The Capetian–Plantagenet rivalry was a series of conflicts and disputes that covered a period of 100 years (1159–1259) during which the House of Capet, rulers of the Kingdom of France, fought the House of Plantagenet (also known as the House of Anjou), rulers of the Kingdom of England, over the Plantagenet-held Angevin Empire which at its peak covered around half of the territory within France. Some historians refer to this series of events as the "First Hundred Years' War".
The conflict can be seen primarily as a French one as both dynasties were French and the nobles and foot soldiers of the Plantagenet armies were often of French origin, especially when the fighting occurred within France. During the rivalry, the continental possessions of the Kings of England were considered to be more important than their insular ones, covering an area significantly greater than the territories controlled by the Kings of France who, however, were the overlords of the former. Indeed, while the Capetian's nominal suzerainty extended far beyond the small domain of Île-de-France, the actual power they held over many of their vassals, including the Plantagenets, was weak. These reasons, in combination with the Plantagenet's hold on the sovereign kingdom of England adding to the strength of the Plantagenets, can be seen the primary reasons for the rivalry.
The rivalry between the two dynasties and the many wars that came with it led to the gradual "reconquest" by the Capetians of most of their own kingdom, and later, the Plantagenet's attempts at retaking what they believed to be their rightful claims in western France. After the conflict ended with the Treaty of Paris in 1259, the English kings, through their few remaining possessions on the continent, would remain vassals to the French kings. The Capetians were also able to consolidate their power, making the kingdom of France the wealthiest and most powerful state in medieval Western Europe.
The rise of the Angevin Empire under Henry II
In 1150, Henry II received the Duchy of Normandy from his father Geoffrey V, Count of Anjou, and when he died in 1151, he became Count of Anjou and Maine.
On May 18, 1152, he became Duke of Aquitaine in right of his wife by marrying Eleanor of Aquitaine in Poitiers after her first marriage with the King of France Louis VII the Younger was annulled at the Council of Beaugency. Several children were born of this marriage.
On November 6, 1153, by the Treaty of Wallingford (or Treaty of Winchester), he was recognized as the successor of King Stephen of England (Stephen of Blois before his accession to the throne). When the latter died on October 25, 1154, he ascended the throne of England under the name of Henry II. On Sunday, December 19, he was crowned at Westminster Abbey.
Henry introduced some religious and legislative reforms. In 1155, he appointed Thomas Becket chancellor.
In 1156, he seized the viscounty of Thouars, thereby controlling communications between the northwest and south-west France.
In 1159, continuing his expansionist policy, he besieged Toulouse with the help of Ramon Berenguer IV, Count of Barcelona and prince of Aragon. Louis VII came to the aid of his brother-in-law Raymond V, Count of Toulouse. Henry II withdrew, annexing part of Quercy and Cahors.
Throughout the early 1160s, Henry II steadily increased his influence in the Duchy of Brittany until he forced Duke Conan IV to abdicate in favor of his daughter Constance, Duchess of Brittany whom Henry promptly married to his own son Geoffrey II, Duke of Brittany, placing the duchy firmly within the Angevin sphere of control. Henry II would rule Brittany until the children came of age and thereafter, indirectly, via subordinates.
Although Henry II wielded much stronger authority within his lands and commanded far greater resources than his Capetian rivals, there was a considerable division in his territories between his sons. Eager to inherit, his three eldest sons rebelled against him with the help of king Louis VII of France. Henry emerged victorious and cemented his status in Western Europe.