Nicomedian Empire (Seven Roman States)

Roman Background and Foundation of the Tetrarchy
The army of the Roman Empire succeeded in conquering a vast collection of territories covering the entire Mediterranean region and much of Western Europe. These territories consisted of many different cultural groups, ranging from primitive to highly sophisticated. Generally speaking, the eastern Mediterranean provinces were more urbanized and socially developed, having previously been united under the Macedonian Empire and Hellenized by the influence of Greek culture. In contrast, the western regions had mostly remained independent from any single cultural or political authority, and were still largely rural and less developed. This distinction between the established Hellenized East and the younger Latinized West persisted and became increasingly important in later centuries.

Under the reign of the Emperor Diocletian, the political division of the Roman Empire began. In 285, he promoted Maximian to the rank of Augustus (Emperor) and gave him control of the Western regions of the Empire. In 293, Galerius and Constantius Chlorus were appointed as their subordinates (Caesars), creating the First Tetrarchy. This system effectively divided the empire into four major regions and created separate capitals besides Rome as a way to avoid the civil unrest that had marked the 3rd century. In the West, the capitals were Maximian's Mediolanum (now Milan) and Constantius' Trier. In the East, the capitals were Sirmium and Nicomedia. On 1 May 305, the two senior Augusti stepped down, and their respective Caesars were promoted to Augusti and appointed two new Caesars, thus creating the Second Tetrarchy.

These men were from the Roman province of Illyria, several in the city of Sirmium, which would become one of the four capitals under this system. From the time of Domitian (81–96), when over half the Roman army was deployed in the Danubian regions, the Illyrian provinces had been the most important recruiting ground of the auxilia and later the legions. In the 3rd century, Romanised Illyrians came to dominate the army's senior officer echelons. Ultimately, the Illyrian officer class seized control of the state itself.

The four Tetrarchs based themselves not at Rome but in other cities closer to the frontiers, mainly intended as headquarters for the defence of the empire against bordering rivals (notably Sassanian Persia) and barbarians (mainly Germanic, and an endless procession from the eastern steppe; many nomadic or elsewhere chased tribes) at the Rhine and Danube.

Reign of Maximinus Daia
Gaius Valerius Galerius Maximinus (c. 20 November 270 – July or August 313), commonly known as Maximinus Daia or Daza, or Maximinus II, ruled as a Roman Emperor from 308 to 313. He was born of peasant stock to the half sister of the emperor Galerius near their family lands around Felix Romuliana; a rural area then in the Danubian region of Moesia, now Eastern Serbia.

He rose to high distinction after he had joined the army, and in 305 he was adopted by his maternal uncle Galerius and raised to the rank of caesar, with the government of Syria and Egypt. In 308, after the elevation of Licinius to Augustus, Maximinus and Constantine were declared filii Augustorum ("sons of the Augusti"), but Maximinus probably started styling himself after Augustus during a campaign against the Sassanids in 310. On the death of Galerius, in 311, Maximinus divided the Eastern Empire between Licinius and himself, keeping the rich and fertile heartlands of Anatolia and Egypt for himself and leaving Licinius with the Balkans. This division of the Eastern Roman Empire is often said to mark the founding of the two distinct kingdoms, Nicomedia and Pannonia. When Licinius and Constantine began to make common cause with one another, Maximinus entered into a secret alliance with the usurper Caesar Maxentius, who controlled Italy.

Maximinus chose Nicomedia in northwestern Asia Minor (modern Izmit in Turkestan) as his capital, an excellent base for defence against invasion from the Balkans and Persia's Sassanid Empire.