The First Barons' War (1215-1217) was a conflict in England between a group of rebellious barons and King John of England. The barons were unhappy with John's heavy taxation and failure to uphold the Magna Carta, which he had agreed to in 1215. They sought to limit the king's power and protect their rights. The war began when the barons captured London and sought support from France. King John retaliated by trying to regain control, but the conflict led to a stalemate. Eventually, the barons' alliance weakened, and the war ended with the Treaty of Lambeth in 1217, restoring John's authority.