The Twenty Years' War was primarily a struggle between Wessex and France for the right to inherit Normandy. However it soon became a general struggle for how much authority France should actually have over the old kingdom of Francia.
In September 1206 the last Rolloian Duke of Normandy, Robert III, died. His nearest relative, his cousin Matilda, was Queen of Wessex, ruling joinly with her husband Theobald of Blois. Their son Henry was quickly appointed Duke and was confirmed in a ceremony in Rouen in March 1207. However, as a minor it would be his father Theobald who took over the day to day running of the duchy, receiving homage from the Norman barons.
However the King of France, John I, felt affronted that the act was signed and sealed without his say-so. Asking Theobald and Henry to pay him homage Theobald saw a chance to rid his own personal territory of French interference and refused, saying that he paid homage to the Kingdom of Wessex not Francia.
Incensed, John declared Theobald's rule in both Normandy and Blois to be forfeit. John had been trying to recover France's authority which had been squandered in civil strife and badly managed wars against Anglia and the Low Countries in the 12th century.
France at first made good headway. Blois and Chartres fell embarrassingly quickly and the Norman lords were soon barricading themselves up, refusing Theobald's calls to muster and face the French on the battlefield. In this the French were ably assisted by Brittany, long at odds with both Normandy and Wessex and in no mood to allow their unification. Maine meanwhile was worried about being squeezed in the middle and broadly supported Wessex in Normandy, this merely meant that it was attacked and occupied from three sides with France gaining the upper hand here too. However Wessex slowly amassed a quality army in Normandy, including its lethal longbows. A series of victories over France secured Normandy though the armies were soon bogged down in sieges in Maine and Blois.
If France did not have its plate full already in 1209 it unwisely heeded a call from the pope to crusade against the Cathar heretics and their protectors in Occitaine. While the crusaders campaigned against Wessex for several years papal impatience finally pushed them southward in 1211. Its main enemies, Auvergne and Aragon defying a papal ban, had been given ample time to prepare their forces in the interim and mauled the French as they journeyed southward. The need to maintain forces in the north fatally undermined the strength of those sent southward for the Albigensian War and the defeat locked it almost permanently out of southern Francia. Meanwhile the death or imprisonment of many of France's finest knights crippled the war in the north, too and, severely short of funds the crown could not pay the ransoms Aragon demanded. They would eventually be released with Papal diplomacy but the war dragged on until 1218 sucking up valuable money and manpower that could have been better used against Wessex.
In 1214 Theobald died. Henry II, even though he was barely 14 years old, had distinguished himself in several battles and had earned the respect and trust of the majority of his lords. Unwilling to allow his mother to rule in his name he made swift moves to imprison her 'for treason', fighting a minor revolt amongst his nobility and using their confiscated assets to build a war chest. Showing some flair for diplomacy he offered Brittany the chance to divide Maine up and Conan V accepted the offer. Both would campaign to expel the French from the county.
Following the collapse of Maine in 1218 Conan V of Brittany secured his 'prize'; the hand of the heiress Erembourg, effectively ensuring Maine (or at least its Western portion) would become a Breton possession. Charles V dangled the promise of recognising the inheritance and Conan took the bait, confirming peace, paying homage and promptly turning his armies back on Wessex.
In 1220 the Wessex barons revolted tired of the onerous taxes being imposed on them for the war effort. This, lingering resentment over the succession crisis and revolt in the Welsh marches stirred passions and meant Henry could not see through his ambitious plans for 1220-21. It took 18 months before Henry II bowed to the inevitable; he simply could not fight his own nobles and his enemies on the continent at the same time. He signed away considerable privileges in the Magna Carta agreement but secured the funds required to shore up the defences. The 2nd Battle of Dreux (May 1224) gave Normandy breathing space once more and Charles V's death in 1225 appeared to give Henry the upper hand. However Charles successor, Louis VIII, squeezed one more victory in before the end of the year and Henry was forced to retreat to Rouen with only a quarter of the men he had set out with.
In early 1226 the French army forced its way into Normandy once more and, pressured by his lords, Henry II finally paid homage to Louis VIII. In return he was invested with the Duchy of Normandy but had to accept the loss of Blois. But this did not see a true end to the conflict. Brittany and Wessex would continue to fight repeatedly over Maine of which Norman barons still occupied the eastern end.
Despite Louis VIII's claim of victory in reality France had been irrecoverably damaged. Instead of numerous small and easily dominated neighbours it was now ringed to the north by three large and capable powers; Brittany-Maine, Wessex-Normany and Anglia. To the south it had been comprehensively thrown out of Occitaine. Louis' brother Philip III would be forced into his own Magna Carta agreement to secure the succession of his only child, Catherine in 1247.
Henry II tried to repeal the Wessexian Magna Carta in 1229 and again 1243-45. On both occasions he failed, however was able to water it down to at least allow some room for manoeuver.