第一次男爵战争（First Barons' War）was an armed military conflict fought in the Kingdom of England from 1215 to 1217. Like several other wars throughout history, it was another theater of the endless conflict between the Assassins and Templars.
In this war, a group of rebellious English barons, supported by the French Army, instigated a civil war against King John after the monarch refused to accept the terms of the newly-crafted Magna Carta agreement. However, the war quickly evolved into an attempt by the French prince Louis VIII to gain English lands.
The Assassins, led by Robert Fitzwalter, fought for the French-aligned Barons while the Templars used their influence to help the King. Additionally, operating independently from the Assassins, Master Assassin William of Cassingham chose to fight for King John and his son, Prince Henry.
Although Louis managed to seize several English towns during the war, his barons eventually defected back to the English side after King John's death, leaving Louis without support and unable to secure his hold on England. On 11 September 1217, Louis relinquished the ground he had won during the war and returned to France, with the conflict ending in a stalemate.