Timeline: 4th Century (Gaul Rising)

Building Relations With Arab Tribes
The Osrinian War had cost Rome an important ally, Armenia. Persia had forced Armenia to agree to be neutral in all Perso-Roman conflicts. The war had also secured the independence of Osroene, a former Roman province and an ally of Persia. Meanwhile, the Arab kingdom of Lakhm was steadily expanding and beginning to exert influence beyond its borders. These things left Rome diplomatically isolated and with an enemy right at its border in the Middle East.

Starting in 301, Roman Emperor Messalla began a project to build trade and diplomatic relations with tribes in northern Arabia. His hope was to eventually gain at least one ally for Rome to counterbalance Osroene.

Reconquest of Cappadocia
Since 298, Messalla had been taking steps to rapidly rebuild the Roman army so that he could launch an invasion of the breakaway province of Cappadocia. These efforts were a success: between 298 and 304, approximately 30,000 men enlisted. In 304, Messalla, Maximian, and several prominent generals agreed that the time was right to invade Cappadocia.

The invasion began in May 304, with 40,000 Roman soldiers participating. Messalla opted for a two-front invasion. 25,000 of the participating men would invade from the southwest, and the remaining 15,000 would stage a naval invasion from the north. Both groups were able to break through the Cappadocian defenses. After winning the initial battles, the Romans steadily gained territory. In October, the governor of Cappadocia surrendered and recognized Messalla's authority.

After Rome retook Cappadocia, the northern part of the province was given to the province of Pontus, and the eastern half was made into a separate province called Armenia Minor. The provinces were added to the Dominium Caesaris. The governor of Cappadocia and most of his important civil and military officials were allowed to retire.

Gallic Reaction
Word of the Roman reconquest of Cappadocia did not reach Colonia Agrippina until a year after the fact. By 302, Tetricus and the Gallic Senate did know that Messalla was rapidly rebuilding the Roman army. Tetricus and the Senate agreed that the appropriate response was to take steps to expand the Gallic army.

The Assassination of Tetricus
Upon being elected First Citizen of Gaul, Tetricus had designated Carausius as his successor, but only to win the support of senators who would have otherwise supported Carausius. Throughout his reign, Tetricus consistently marginalized not only Carausius but also his closest associates, and reserved important civil and military offices for Postumians. The purpose was to make it so that if Carausius did indeed succeed Tetricus, he would lack meaningful leadership experience, and be heavily dependent on Postumians who had been groomed to exercise the real power.

By 304, Carausius had decided that the only way he could ever hope to rule without being a puppet of allies of Tetricus was by having Tetricus assassinated and seizing the throne by force. Now at this time, Carausius was governor of the province of Asturia (in northern Spain). On March 9, he sent a letter to a longtime friend of his named Sextus Aurelius Allectus. At that time, Allectus held a minor military command in Belgica (specifically, he was a centurion), so he was the closest person to Colonia Agrippina whom Carausius trusted. In the letter, Carausius promised that if Allectus were to participate in the assassination, he would be rewarded with an important role in the government under Carausius, and possibly even be chosen to succeed Carausius; assurred Allectus that if he declined to participate, it would not be held against him; and asked Allectus to tell him whether or not he was willing to oversee the assassination. Allectus received the letter on April 27; and on May 2, Allectus sent a reply in which he informed Carausius that he was willing to participate.

Planning the assassination was Allectus' responsibility, since he was much closer to Colonia Agrippina than Carausius. He had several friends and associates in Colonia Agrippina. Without revealing his intent to have the emperor murdered, he began asking those contacts about the people they knew and regularly dealt with. Specifically, he asked if any of them knew men who could be considered highly unscrupulous or men who were desperate to escape debt or poverty. He also sent a couple of men under his command to visit nearby prisons to learn about the people who being held there. He also noting which of the men in his unit stood out as particularly ambitious. Allectus' goal was to find several people who easily could be bribed into participating in the assassination.

Over the summer of 304, the details of the plan began to take shape. Several men would be trained in archery. Once Allectus was satisfied that their skills as archers were good enough, a date for the assassination would be selected. On that date, the assassins would hide near the building where the Senate met, and shoot as he was about to enter. Thereafter, the assassins would be hidden. Once Carausius had secured his authority over the Gallic Empire, the assassins would received whatever rewards they were promised.

At the same time that the plan was developing, however, the activities of Allectus aroused the suspicions of his immediate superior, the commander of the cohort that Allectus' unit was a part of. He noticed that men under Allectus' command would sometimes be absent. Also, the volume of mail to and from Allectus began to increase. At the same time, jailers in Belgica were beginning to wonder why Roman soldiers were wanting to learn everything they could about the current prisoners, as well as prisoners who had been released. In spite of this, there was nothing to clearly suggest that Allectus was involved in any conspiracy. Allectus had considered bribing the guards to release one or two prisoners, but he eventually chose not to do that, feeling that that would implicate him in the killing of the emperor.

On July 6, Carausius received Allectus' letter in which Allectus informed Carausius that he was willing to oversee the assassination of the emperor. After Carausius received and read the letter, he immediately began preparing to leave Lucus Asturum (OTL Llanera) for Colonia Agrippina, and he left on July 20.

The assassination of Tetricus took place on the morning of September 8, 304. On that day, Tetricus was on his way to the Senate building. Five men were hiding at different points near the entrance. Two were criminals who had previously served time in prison, one was a farmer who was deeply in debt, and two were low-ranking but ambitious soldiers in Allectus' unit. As Tetricus began to walk up the steps that led to the door, all five of the assassins shot arrows at him. Each assassin had dipped his arrows in poison. Three of the arrows hit Tetricus, and he fell to the ground. Each of the assassins shot a second arrow; and one of those arrows hit him, and three hit his bodyguards. Thereafter, all five of the assassins fled the scene. Tetricus and the two bodyguards of his who were shot died within minutes.

The First Gallic Civil War
Main article: First Gallic Civil War

Carausius arrived in Colonia Agrippina on September 18, 304 and immediately assumed the throne. Immediately, suspicion arose that Carausius was involved in the assassination of Tetricus. Also, the Postumians anticipated that Carausius would either purge them from the Senate and the military or create new Senate seats for his partisans to dilute their influence. The Postumians secured an agreement from Carausius to not alter the membership of the Senate for six months. They used this time to investigate the assassination of Tetricus and garner votes to depose Carausius.

By February 305, the Postumians had uncovered evidence that Carausius had orchestrated the killing of Tetricus. Also by this time, Carausius had had several senators assassinated and bribed and intimidated several others into doing his bidding. The Postumians knew it was time to act. They persuaded the most senior senator to call a meeting on February 16 after dark for the purpose of deposing Carausius without giving the Princepist faction any advance notice. To justify holding the vote, the senior senator adopted an interpretation of the rules of proceedings that essentially gutted them. Seventy-five senators walked out of the meeting just before the vote was held. The remaining senators voted to depose Carausius. The Senate appointed Aurelius Arpagius, a general who had fought in the Amasigan War and the governor of the province of Massiliensis, as the new First Citizen; and the Senate also declared that Tetricus' son, known as Tetricus the Younger, would become First Citizen in the event of Arpagius' death.

Carausius refused to recognize the Senate's vote to replace him, and he declared the senators who had voted to depose him to be expelled from the Senate. Likewise, the senior senator refused to recognize the dismissals made by Carausius. The Senate itself split into two bodies: one consisting of the senators who had voted to depose Carausius, and the other composed of the senators who had either walked out of the overnight meeting or voted against deposing him. For several days, the situation in Colonia Agrippina was tense but nonviolent; but on February 24, 305, Carausius sent troops loyal to him to arrest the pro-Arpagius Senate. Most of the pro-Arpagius senators avoided capture; but by February 26, they were forced to flee Colonia Agrippina. This marked the beginning of the First Gallic Civil War.

During March 305, military commanders and civil officials chose sides as word of the events of late February spread throughout the empire. The pro-Arpagius Senate moved to Augusta Treverorum (OTL Trier), while Carausius and his Senate relocated to Turnacum (OTL Tournai). Arpagius and Tetricus the Younger learned about the Senate vote and the outbreak of the war in late March. By April, most of northern and western Gaul, a few areas in Spain, and the province of Britannia Superior had sided with Carausius; while the rest of the empire had accepted Arpagius.

During April, Arpagius prepared for a campaign to retake Colonia Agrippina and Tetricus the Younger planned a campaign to seize a strip of land running right through the middle of Carausius' core territory, while Carausius planned an effort to take Augusta Treverorum. Meanwhile, supporters of Arpagius in Spain took the initiative of beginning a campaign to take over the areas loyal to Carausius. Both Carausius and Tetricus the Younger began their campaigns in late April.

The forces that Carausius sent to take Augusta Treverorum failed. By the middle of May, Arpagius had learned about the presence of the forces attempting to take the city, and he and the legions that were with him intercepted them. The pro-Carausius units were wiped out. Then in June, Arpagius led several legions to take Colonia Agrippina and the surrounding areas. He hoped that taking the empire's capital would lead to the death or arrest of Carausius and the end of the war. He only learned after taking over Colonia Agrippina that Carausius had long since left. Several weeks after the Second Battle of Colonia Agrippina, Arpagius sent troops to advance westward. These forces were able to gain control of an area that almost extended to the Mosa River (OTL Meuse).

Tetricus the Younger spent the summer of 305 trying to take over a strip of land running through the middle of northern Gaul. The purpose of this campaign was to cut Carausius off from half of his territory. Tetricus the Younger did gradually advance through northern Gaul, but his forces were repeatedly intercepted by forces loyal to Carausius.

The British theatre of the war began in June 305, when the governor of Britannia Inferior, Flavius Dannicus, and the dux of Britain, Julius Proculus, agreed to take the initiative of invading Britannia Superior. Dannicus and Proculus both supported Arpagius; and their goal was to remove the governor of Britannia Superior, named Lucius Durus, and then send troops to aid Arpagius and Tetricus the Younger in Gaul. The effort to take Britannia Superior culminated in the Battle of Londinium and the Battle of Noviomagus Reginorum, both of which took place in August, and which resulted in the complete takeover of Britannia Superior by Proculus.

In October, Proculus sent several legions to invade the areas in Gaul that Carausius controlled. This led to the end of the war. With the help of the Britannia forces, Tetricus the Younger was able to finish carving a swathe through Carausius' territory, and then win several other cities. Several senators and generals who had supported Carausius became disillusioned with him; and on November 8, they arrested him and ordered all the military units who had sided with Carausius to surrender and recognize Arpagius as First Citizen. Not all of Carausius' supporters obeyed these orders, so the war did not end until January 5, 306.

305 and 306
In September 305, Messalla decided to exploit the fact that the Gallic Empire was in a state of civil war. Rather than attack the Gallic Empire itself, Messalla decided to invade Gaul's allies, the four Germanic states in Pannonia and Noricum. The Roman invasion of Pannonia and Noricum began on September 22, 305. Messalla sent a large invasion force to the area, and the Pannonian and Norican states were caught off-guard. Because of this, the Romans were able to conquer all of Suevia by October 18 and a third of Marcomannia by the end of October. During October, the governments of all four countries sent letters to Arpagius requesting aid.

Arpagius learned about the invasion of Pannonia and Noricum by late November 305. By this time, he had learned about the downfall of Carusius, so he believed that he could afford to aid the Germanic states. Arpagius promptly sent two legions and four auxiliary units to assist the Pannonian and Norican armies. He also sent troops to invade the Roman Empire itself. Meanwhile, the governor of Raetia took the initiative of sending three auxiliary units into Alamannia (but only Alamannia).

The Gallic units that were deployed to Pannonia and Noricum arrived in January 306. By that time, the Romans had taken two thirds of Marcomannia and almost half of Quadium. Alamannia, on the other hand, had remained mostly unconquered, thanks in part to the help from the Gallic units that had already been sent there.

At the same time that Gallic troops were entering Pannonia and Noricum, four legions and eight auxiliary units set sail from southern Gaul to invade the Roman provinces of Numidia, Mauritania Caesariensis, and Mauritania Sitifiensis; and another two legions and four auxiliary units sailed to Corsica. In February 306, two legions and six auxiliary units from Spain received and carried out orders to invade Mauritania Tingitana. Meanwhile, two additional legions were sent to defend Gaul's already heavily protected border with Italy. Altogether, Arpagius had chosen to invest almost half of the Gallic Empire's total military strength in the war with Rome.

Arpagius' strategy paid off. The presence of the Gallic forces in Pannonia and Noricum enabled the local armies to hold the line against the Romans and even retake a few areas, which bought time for the invasion of Corsica and the west African provinces to proceed. In the Roman provinces that the Gallic Empire invaded, the Gallic forces outnumbered the Roman forces; and in Corsica and Numidia, the local Roman forces were caught completely by surprise.

The general purpose of the invasion of the Roman Empire was to force Messalla to withdraw the troops he had sent to Pannonia and Noricum. Arpagius also intended to permanently annex Mauritania Tingitana and at least temporarily occupy Corsica. The forces sent to Numidia, Mauritania Caesariensis, and Mauritania Sitifiensis were ordered to plunder and destroy as many cities and towns as possible within a month, and then leave.

Messalla learned about the invasion of Corsica first, and he responded by sending troops that were stationed in Italy. After he received word about the invasion of the African provinces, he sent two legions from Italy to northwest Africa. To partially compensate for the absence of troops in Italy, he pulled one legion from Noricum and two auxiliary units from Dalmatia.

By the time the Italian legions arrived in Africa, most of the Gallic forces were already gone. Per orders from Arpagius, one legion and three auxiliary units moved to Mauritania Tingitana to help with the invasion of that province, one legion and the other five auxiliary units sailed to Corsica to bolster the Gallic presence there, and the other two legions retreated to southern Gaul and western Spain. After this, the Romans attempted to retake Mauritania Tingitana and Corsica, but they were repeatedly repelled. In Noricum and Pannonia, help from the Gallic troops had enabled the Germanic states to steadily win back territory from the Roman Empire. Also, the departure of an entire Roman legion made it possible for resistance cells to form in the areas that were occupied. By May, the resistance movement had taken control of several small towns and two relatively large cities. Most of the Roman troops were near the front lines at this point, but the legion commanders were forced to move some cohorts to the interior to deal with the rebellion. The tactics used by troops sent to quell the rebellion only outraged the people and prompted more people to join the uprising. By July, it was clear to the generals commanding the invasion force that they could not fight both the armies of Gaul and the Germanic countries and the resistance cells over an area as wide as the entire Noricum-Pannoia region, so they ordered their forces to retreat to the area south of the Dravus (OTL Drava) River. Even then, Gallic and Germanic forces were still able to chip away at the Roman gains, so the Roman generals soon ordered a retreat to the area south of the Savus (OTL Sava) River.

In late August, Maximian sent one legion under his authority to the Noricum-Pannonia front, another to Corsica, and a third to Mauritana Tingitana. The legion bound for Pannonia and Noricum arrived on October 1, 306. During October and November, the Romans were able to steadily regain territory. The legion that was sent to the Corsica front arrived on October 17. This legion was less successful: not only did the Gallic army still have a large military presence there, but it had also bolstered its strength by bribing hundreds of local Roman soldiers into defecting to the Gallic Empire. News of the arrival Roman of additional Roman forces in Pannonia and Noricum reached Gallic territory by late October. On November 3, the dux of one of the Gallic Empire's eastern military districts took the initiative of sending a legion to the area. This legion arrived on November 26. The Gallic and Germanic armies won several victories during the first half of December, but then a stalemate developed. Meanwhile, the legion sent from the Dominium Caesaris to Mauritania Tingitana arrived on December 4. That legion's presence there did not help the Romans: the Gallic presence was simply too strong.

307
On January 4, 307, Messalla ordered a complete withdrawal of Roman troops from the area north of the Savus River. The legion from the Dominium Caesaris was to return there, while the other two legions were to hold the line at the Savus. Messalla also ordered one of the two legions he had sent to Africa to go to Corsica to retake the island. The Roman legion that was redeployed to the Corsica front arrived at northern Sardinia on January 26. On February 8, it and three other legions staged a massive attack on Corsica. The Gallic forces won, but their victory was costly. On February 14, the general who had been acting as governor of Corsica sent a letter to Arpagius asking him to allow a full retreat from Corsica. After another pyrrhic victory against Roman forces on March 1, the governor of Corsica decided to abandon Corsica without waiting for Messalla's approval. By the time the invasion force pulled out of Corsica, over a quarter of its participants had died.

After the Gallic withdrawal from Corsica, half the invasion force went to Mauritania Tingitana, and the other half went to Gaul's pre-war territories. For several months after this, the Romans repeatedly made failed attempts to retake Mauritania Tingitana, while the Gallic and Germanic armies failed to take anything south of the Savus. Finally, on June 15, 307, Messalla sent a letter to Arpagius to offer terms for an armistice: Messalla was willing to accept the loss of Mauritania Tingitana, provided Arpagius were to accept Roman control over the parts of Pannonia and Noricum south of the Savus. While awaiting a reply from Arpagius, Messalla ordered his forces to hold the line at the Savus and the border of Mauritana Tingitana, but not attempt to move beyond those boundaries. This effectively marked the end of the war. On September 4, Messalla received a letter from Arpagius in which Arpagius stated that he accepted Messalla's terms.

The African Rebellion
For several years before the Third Gallo-Roman War, discontent with Messalla had started to appear in the African provinces. The people of the African provinces (especially Numidia and the Mauritanian provinces) were beginning to believe that Messalla had neglected them relative to the rest of the empire. After the war with Gaul, the dissatisfaction with Messalla grew rapidly in northwest Africa.

By March 308, riots targeting local Roman officials were happening in many cities in northwest Africa, and militias were beginning to form. During June 308, militias seized control of three cities in Mauritania Caesariensis and one city in Numidia. Some army units joined up with the militias, while others tried to crush the rebellion. On June 13, 308, Gaius Annius Anullinus, the governor of Numidia, declared himself Supreme Senator (senior co-emperor) and his son Caesar (junior co-emperor). After this, several legions quickly turned against Messalla.

Anullinus promptly sent two legions and three auxiliary units to invade Rome. These legions left Africa on June 28 and arrived in central Italy on July 10. The invasion of Rome took place on July 13. There was only one legion protecting the city, and with that legion outnumbered and caught off-guard, the city quickly fell to Anullinus' forces. Messalla managed to escape to Spoletum (OTL Spoleto). From there, he summoned troops to Italy, informed Maximian of his situation, and ordered generals loyal to him in Africa to invade Numidia. The invasion of Numidia began on August 12. On August 16, Messalla led a campaign to retake Rome. By August 17, Anullinus' forces had been driven out of Rome, and pro-Messalla forces were closing in on Cirta (OTL Constantine), the capital of Numidia. On August 20, a legion in Mauritania Sitifensis that had remained loyal to Messalla seized control of Sitifis (OTL Sétif), the provincial capital. By then, most of the pro-Anullinus forces in Numidia had capitulated. On August 21, pro-Messalla forces took Cirta and killed both Anullinus and his son.

Messalla was injured in the battle to retake Rome from Anullinus. The wound was treated, but it became infected. Messalla died from the infection on August 24, 308. The local officials quickly sent the news of Messalla's death to Maximian, who was in Thessalonica. Maximian received the news on October 10. Maximian's first act as Supreme Senator was to make his son Maxentius the new Caesar of the Dominium Caesaris. Maximian left Thessalonica for Rome on October 31, and he arrived on December 17.

Consolidating Power
Arpagius learned about the coup against Carausius on November 18, 305. On November 27, he ordered Carausius to be transported to Colonia Agrippina. Carausius arrived on December 27. On January 20, 306, Carausius was put on trial for orchestrating the murder of Tetricus the Elder and leading a rebellion against Arpagius. The trial ended on January 24. Carausius was found guilty and sentenced to death, and this sentence was carried out on January 30.

Arpagius pardoned the senators and generals who had led the coup against Carausius, and he allowed them to keep their respective Senate seats and military commands. The remaining pro-Carausius senators were expelled from the Senate and put on trial for aiding the rebellion, and all but nine of them were convicted; and those convicted received sentences ranging from ten years in prison to death. Arpagius' treatment of generals and other high-ranking military officers who had supported Carausius was mixed. Some of the ones who had not actually engaged in combat against forces loyal to Arpagius were allowed to retain their ranks; but most within this category were demoted, and a few were dismissed from the army. All officers above the rank of centurion — as well as some centurions — who had actively participated in combat with supporters of Arpagius were expelled from the army and put on trial for rebellion against Arpagius; and most of these were convicted and sentenced to either life in prison or death. All of the lower-ranking soldiers who had fought on Carausius' behalf were pardoned All the provincial governors and duces who had sided with Carausius were put on trial, found guilty, and sentenced to either life in prison or death. All of these proceedings were carried out between January and May of 306.

Victory for Arpagius in the civil war meant victory for the Postumians. All of the Princepist senators had sided with Carausius when the war began, and Arpagius replaced all of them with Postumians, except for five who participated in the coup that ended the war. Most of the new senators were Postumians, and those that were not Postumians were affiliated with the "strong leadership" bloc. Likewise, most of the military commanders Arpagius appointed to replace the ones who had supported Carausius were also Postumians. All of the pro-Carausius duces and governors were replaced with Postumians.

Integration of Mauritania Tingitana into the Gallic Empire
Mauritania Tingitana fell to the Gallic Empire with little resistance. The governor and military commanders knew they were severely outnumbered and surrendered within a matter of weeks. The process of integrating Mauritania Tingitana into the Gallic Empire began in April 306. The governor and key military officers and civil administrators were removed from office and placed under house arrest for four months. One of the Gallic generals assumed the governorship.

Arpagius had ordered the Gallic forces to ensure that the everyday lives of the locals were not affected by the change in administration. The invading generals had troops patrol the streets of all the major cities constantly and the countryside as much as practical, but ordered their troops to leave the locals alone so long as they did not cause trouble. Much of the provincial bureaucracy continued to be staffed by locals.

During April, the Gallic governor offered the Roman soldiers the opportunity to join the Gallic army, and many Roman soldiers accepted the offer and were integrated into Gallic units. Also, the governor began working with the governor of Baetica to the north and the governor of Amasiga to the south to build trade links, in order to integrate Mauritania Tingitana into the Gallic Empire's economy. In August 306, the governor began gradually introducing the Gallic currency into the province, and the Gallic currency had fully replaced the Roman currency by 308.

Relocation of the Capital
In November 308, Arpagius proposed to the Senate that a new capital city be chosen for the empire. Colonia Agrippina was located at the eastern edge of the empire, and Arpagius believed it would be good for the capital to be moved to a city in the interior of Gaul, as moving the capital westward would allow for more efficient contact between the central government and the Spanish and African provinces. After several months of delibation, the Senate voted to designate Tricassium (OTL Troyes) as the new Gallic capital city on March 25, 309.

The move to Tricassium would not happen for almost a decade. After the Senate chose Tricassium, they commissioned architects to design a new imperial palace. Construction of the new palace began in March 310. Arpagius and the Senate left Colonia Agrippina on September 6, 318 and arrived in Tricassium on September 19, 318, although the construction of the new imperial palace would not be complete until July 319. The various bureaucracies of the empire moved their headquarters from Colonia Agrippina to Tricassium between November 318 and May 320.

Military Reform
After the Gallic Empire broke away from the Roman Empire, its military had been very similar the Roman military. Arpagius carried out two major reforms during his reign.

Arpagius announced his first reform came in July 308. Up until that point, legions had consisted of ten cohorts, which in turn consisted of six centuariae (nominally groups of one hundred soldiers, but in practice groups of eighty); plus a small cavalry contingent. The centuriae were to be replaced with quinquagintaniae (groups of fifty), and the commander of a quinquagintania would be called a quinquagintanion. Cohorts would consist of consist of eight quinquagintaniae. Legions would consist of six cohorts, plus a hundred-strong cavalry unit. Though the legions would be smaller, they would be more mobile.

The second major reform was announced in March 311. The Gallic Empire had always had a navy, which was formed from local branches of the Roman navy when the Gallic Empire became independent; but the Roman Empire had long neglected its navy, and the Gallic Empire had inherited this tendency of neglect. Arpagius wanted to dramatically expand the Gallic navy from ten thousand men to fifty thousand. This expansion took years to accomplish.

Arpagius also expanded the overall size of the Gallic army. On the eve of the First Gallic Civil War, the army had approximately 160,000 men. After the Third Gallo-Roman War, that number had declined by 15,000. In the decade after the wars, Arpagius took steps to expand the army to 200,000 men.

The Reign of Maximian
The ascent of Maximian to senior emperorship increased the importance of Diocles Valerius, a longtime friend of Diocles who had been very influential within the Roman government ever since Messalla had appointed Maximian Caesar. Diocles had long been a trusted advisor to both Messalla and Maximian, but he had always had less influence over Messalla than Maximian. The death of Messalla essentially led to Diocles becoming Roman emperor in everything but name. During Maximian's time as Supreme Senator, most policies that those two emperors enacted were either formulated or heavily influenced by Diocles.

Personality Cult
For several years before Messalla's death, Diocles had been building a personality cult centered around Messalla and to a lesser extent Maximian. Messalla neither endorsed nor opposed this during his lifetime. After Messalla died, the personality cult intensified. Rome had had an imperial cult since the time of Augustus, but what Diocles advocated was for the imperial cult to be augmented to portray Messalla as greater than all previous emperors except Augustus.

Diocles' propaganda portrayed Messalla as the personification of the values that had made Rome great in the past, the representative of the gods on earth during his lifetime, and having become a god after his death. Similarly, the system of dual rulership was described as ordained by the gods, the appointment of Maximian as Caesar was described as being directed by the gods, and Maximian and all subsequent successors of Messalla were portrayed as inheritors of the role of intermediary between the gods and the people.

Maximian and Maxentius soon embraced the additions to the imperial cult. In accordance with this idea, the two emperors ordered huge palaces to be built for themselves, began wearing ornate purple robes and golden crowns, and instituted elaborate court ceremonies. Temples began to be built in honor of Messalla. May 30, the anniversary of the end of the Great Roman Civil War, was declared the Feast of Messalla.

Economic Policies
By the time Messalla died, the Roman economy was still recovering from the wars and rebellions of the 270s and 280s. The public works program had done little to lift people out of poverty, and the currency reform had failed to stop inflation. Out of frustration, Diocles began formulating more radical economic policies, and the emperors happily accepted most of Diocles' proposals.

Monetary Policies
The new denominations of coins that were introduced under Messalla's authority were still less common than the older, debased coins by the time Messalla died. Even those who had significant amounts of the new coins tended to use them only to pay for significant transactions, and used debased coins to finance mundane transactions. To complicate matters, Messalla had never specified a rate at which debased coins could be exchanged for new ones; and different quantities per capita of follii (bronze coins) and copper coins were minted in different jurisdictions, which meant that the values of the different coins in practice deviated from the official standard in most parts of the empire.

In April 309, Maximian issued an edict titled the Edict on Coinage, which outlawed the debased coins and ordered that the debased coins be confiscated and replaced by minting coins of the new denominations by the end of the year. The edict also declared one laureatus equal to one hundred antoniniani or two hundred denarii.

What Maximian (and Diocles, who proposed the edict) did not know was that the actual value of the new coins in terms of the old ones was higher than the official rate in some places and lower than others. In places where there were not enough new coins, either holders of the old currency were undercompensated for the confiscation, or more of the new coins than necessary to meet the prescribed ratio were minted. Another issue was that the majority of the new coins was of the bronze and copper denominations, and very few of the new coins were solidi (gold coins). This exacerbated the growing problem of the devaluation of the bronze and copper coins relative to the gold and silver coins. Another problem was that the amount of the new currency an individual was entitled to receive was deducted by the amount that he or she already possessed, which meant that much of the old currency was taken without compensation. Since the old currency was still used for everyday transactions, this uncompensated confiscation amounted to losses of wealth for the parties affected. Also, in some provinces, some individuals who had considerably more of the new currency than the equivalent amount of the old currency had the excess amount of the new currency taken from them.

During 309 and 310, wages and prices for every kind of good or service fluctuated wildly as the old coins were replaced people struggled to correctly value things exclusively in terms of the new currency. At Diocles' suggestion, Maximiam responded by issuing the Edict on Acceptable Prices, which set price ceilings and price floors for more than two thousand products and services. As this law was based on ignorance to the law of supply and demand, it led to surpluses and shortages all over the empire.

Taxation
Diocles proposed reforms to the imperial tax system during early 311. Whereas all of Italy had previously been exempted from taxation, Diocles proposed that only Rome and its suburbs be exempt. At the same time, he proposed that Thessalonica, the capital of the Dominium Caesaris, be granted exemption from taxation. Diocles also proposed allowing taxes to be paid in various kinds of goods in lieu of money.

The tax plan was first accepted by Maxentius in May 311. Maximian began implementing the proposal in November 312.

Attempts at Economic Planning
By 314, the price controls were yielding disastrous results. Many people were refusing to sell underpriced goods and unable to buy artificially overpriced goods, and black markets were developing. Diocles proposed requiring governors to collect information about the types of goods and services that were rare in their respective provinces; compel people to enter, remain in, or leave certain trades or occupations; and confiscate and distribute goods that people would not voluntarily sell. This policy was quickly implemented in the Dominium Caesaris and several provinces outside it. Several governors in Italy and Africa pursued an alternate policy: taxing the richest inhabitants of their provinces and redistributing the money and in-kind payments.

Precursors
Christianity had been growing rapidly in the Roman Empire since the 270s. By the 290s, the imperial government began to take notice. In 297, Messalla disqualified Christians from serving as governors or praetorian prefects; and during the 290s, several governors and two praetorian prefects (one of whom was Diocles) carried out purges of the army and bureaucracy within their respective jurisdictions. In 303, Maximian made it illegal to proselytize Christianity within his territory, and Messalla extended this ban throughout the empire a year later. No other actions were taken against Christians at the imperial level for years, although government discrimination steadily intensified in the Dominium Caesaris and various praetorian prefectures and provinces.

Maximian had tolerated discrimination against Christians for years by praetorian prefects and governors for years, but had largely declined to participate himself. The turning point came on May 30, 314, when several Christians protested celebrations of the feast of Messalla. This prompted Maximian to assume a positively anti-Christian stance. In early June, Maximian wrote a letter to Diocles to ask Diocles what measures he thought were appropriate. Diocles, who was preparing to retire, told Maximian that the appropriate model of anti-Christian policies had already been set up in many areas and only needed to adopted and built upon.

The Great Persecution
Although Christians had faced escalating discrimination from the state for years, the Great Persecution is considered to have begun on August 2, 314, when Maximian published the First Edict Against the Christians. This law officially set the minimum penalty for proselytizing Christianity at five years in prison, barred Christians from the army and all other state employment, deprived Christians of the right to petition courts or testify in court, made it illegal for Christians to assemble for worship (with violations punishable by no less than five years in prison), and called for the confiscation and destruction of copies of the Bible and other Christian texts. The edict also instructed the Caesar, praetorian prefects, governors, and other officials to take any actions beyond enforcing its provisions that they deemed necessary to repress Christianity.

As far-reaching as Maximian's edict was, progressively harsher policies were quickly adopted in many parts of the empire, including the Dominium Caesaris. Maxentius raised the minimum prison term for violating the edict to a life sentence, and governors of many provinces both inside and outside Maxentius' realm made violating the edict punishable by death. In February 315, Maxentius ordered the arrest of all Christian clergy within the Dominium Caesaris, and the governors of several provinces outside the Dominium Caesaris followed suit during the subsequent few months. In the summer of 315, provincial governors began subjecting Christians to higher taxes than other citizens owed, and this policy was officially adopted by Maxentius in October 315.

During 316, no additional anti-Christian legislation was enacted. Then in January 317, Maxentius offered amnesty to imprisoned Christian clergy, provided they offered sacrifices to the Roman deities. This offer was extended to Christians imprisoned for assembling for worship or proselytizing in March. Few Christians actually performed the sacrifices: most prisons in the Dominium Caesaris were overcrowded, so the wardens of many prisons simply used Maxentius' proclamation as an opportunity to release most or all of their Christian prisoners, and falsely recorded the sacrifices as having been performed. Finally, in August 317, Maxentius issued a law that all inhabitants of the Roman Empire were required to offer sacrifices to the Roman deities at least twice a year, with the punishment for refusing to do so being death. In January 318, Maximian issued the Second Edict Against the Christians, which made refusing to sacrifice to the Roman deities at least twice a year a capital crime. A year later, Maximian published the Third Edict Against the Christians, which made it illegal for non-Christians to protect Christians from arrest or for local authorities to falsely record required sacrifices as having been performed, with the minimum punishment for either action being ten years in prison.

The Civil War of the Septumvirate
By 320, discontent with Maximian and Maxentius was growing. The consequences of their economic policies had been disastrous. For all the imperial propaganda claiming that Messalla was a god and that Maximian was a representative of the gods, few pagan Romans actually believed these things. The persecution of Christians had failed to wipe out Christianity or even significantly reduce the population of Christians in the empire (and majority of the small decrease in the Christian population that did occur was due to emigration by Christians who lived close to the imperial border). Also, the majority of the pagans did not support the persecution of Christians (which was in fact the reason for Maximian's third edict).

A turning point came on June 7, 320. Ovinius Gallicanus, the governor of the province of Dalmatia announced that he had converted to Christianity, had refused to resign, and would henceforward refuse to enforce the empire's anti-Christian legislation. Between 297 and 314, two other governors had become Christians and immediately resigned, but the governor of Dalmatia chose not to resign because he knew he would be replaced by someone who would enforce the persecutory edicts. Word quickly spread. Several governors of neighboring provinces who were sympathetic to the Christians were impressed with the Dalmatian governor's willingness to defy Maximian. On July 3, the governor of Histria announced that he would end the persecution in his province; and he was followed by the governor of Epirus on July 17 and the governor of Sardeata on July 31. Also, on July 12, the dux of the military district that Dalmatia was a part of announced that he would defy any order to forcibly remove the governor of Dalmatia.

On August 19, 320, the governors of Apulia, Dalmatia, Epirus, Histria, and Sardeata, and a dux from Illyricum and a dud from Haemus (OTL Balkans) met in Salona (OTL Solin). Four days later, they published the Declaration of Salona. In the declaration, they announced that they had formed a seven-member council called the Septumvirate. The Septumvirate would lead a rebellion against Maximian and Maxentius, and then act as a provisional government. The members of this council agreed to end the imperial-level persecution of Christians and prohibit all local persecution, grant Christians equal rights under the law, and repeal the Edict on Acceptable Prices and the mandate for governors to institute economic planning. The members of the Septumvirate agreed to elect a new Supreme Senator after overthrowing Maximian and Maxentius, and then disband.

Promising to repeal the worst of Maximian's economic policies made the Septumvirate very popular. During September in Italy, Africa, and the Balkans, many army units declared allegiance to the Septumvirate, civilians began forming pro-Septumvirate militias, and the governors of Calabria and Dardania defected to the Septumvirate. In late September, Maximian launched an invasion of Septumvirate-controlled provinces. At the same time, militias in many cities and towns in Rome's European and African provinces began fighting to overthrow local officials.

Maximian and Maxentius rapidly lost territory to the Septumvirate due to defections, conquests, and popular uprisings. By February 321, the Septumvirate controlled all of Illyricum, most of southern Italy, northeastern Italy, eastern Sicily, northwestern Haemus, Egypt, Zeugitana, Cilicia, Pontus, parts of Galatia, and Cyprus. Then in early February, a stalemate emerged in Italy, Africa, and the Balkans (although the Septumvirate steadily gained territory in the Levant and Anatolia). This stalemate lasted until late April. On April 22, the residents of Rome rose up against Maximian. By April 25, militias and defecting army units had taken control of several parts of the city, and Maximian had fled for Florentia (OTL Florence). At the same time, Maxentius had begun a large-scale offensive in the Balkans. During May, the Septumvirate gained control of central Italy and western Sicily, but lost parts of Dardania and Epirus. Maxentius' offensive continued in early June, but he was forced to partially pull back when the governor of Galatia surrendered to the Septumvirate.

During June and July, Maximian and Maxentius held their ground against further advances by the Septumvirate and its supporters. Also during this time, Maximian and Maxentius began to ease their persecution of Christians. On June 18, Maximian repealed the Second and Third Edicts Against the Christians (although he did not forbid or even discourage the continued implementation of its provisions by the governors or praetorian prefects who were still loyal to him). On July 9, Maxentius issued an unconditional amnesty for Christians who were imprisoned within the Dominium Caesaris. This, however, fell far short of the full equality under the law that Christians had already been granted in the areas under the Septumvirate's control. Also, Maximian and Maxentius still failed to realize how unpopular their economic policies were and left them unchanged.

On July 28, the praetorian prefect of Mauritania was removed from office in a coup staged by the governor of Mauitania Sitifensis, who then declared himself praetorian prefect and declared allegiance to the Septumvirate. He and his supporters eliminated all resistance within three weeks. On August 15, he sent a legion to invade northern Italy. These soldiers arrived in Italy on August 22. Maximian fled from Florentia on August 27. His plan was to move to Genua (OTL Genoa); but by the time he arrived, on September 3, the local army commanders were already preparing to surrender. They offered to have him hidden and ensure that he would enjoy a comfortable retirement in exchange for his resignation. Maximian accepted the offer.

As news of Maximian's disappearance spread, governors and praetorian prefects who had remained loyal to him rapidly began either surrendering or defecting. Maxentius refused to surrender though. Out of desperation, Maxentius relaxed the price controls on October 5 and repealed the First Edict Against the Christians on October 9, but it was too late. By November 12, Thessalonica was surrounded. On that day, his generals killed him, surrendered to the Septumvirate, and then killed themselves.

Beginning of the Celtic Renaissance
During the 290s, the governors of Aremorica, Lexovia, Aulercia, and Pictonia placed Gaulish on par with Latin as an administrative language. The governor of Belgica made Gaulish an administrative language in 302, and the governor of Senonium would do likewise in 303.

The effects of the policy on administrative languages began to manifest itself in the 320s. By that time, it had become necessary for the provincial elites to be able to understand Gaulish. Government documents were written in both Latin and Gaulish in the Gallic Empire's northern continental provinces. Also, privately-written texts that were originally written in Latin or Greek were being translated into Gaulish. Then in 324, a Belgican author wrote the first book to be originally written in Gaulish (it would not be translated into Latin for more than two decades).

The 320s marked the beginning of the Celtic Renaissance. This would be a time period during which the culture of the northern parts of the Gallic Empire would become less Latin and more Celtic. Although this cultural shift was concentrated in northern and central Gaul, it would later spread to Britannia, and have a limited influence on the empire as a whole.