The First Barons' War (1215–1217) was a civil war in the Kingdom of England in which a group of rebellious major landowners (commonly referred to as barons) led by Robert Fitzwalter waged war against King John of England. The conflict resulted from King John's disastrous wars against King Philip II of France, which led to the collapse of the Angevin Empire, and John's subsequent refusal to accept and abide the "Magna Carta", which he had sealed on 15 June 1215.

The rebellious barons, faced with an uncompromising king, turned to King Philip's son, Prince Louis, who, in 1216, then sailed to England with an army despite his father's disapproval, as well as the Pope's, who subsequently excommunicated him. Louis captured Winchester and soon controlled over half of the English kingdom. He was proclaimed "King of England" in London by the barons, although never actually crowned.

Louis' ambitions of ruling England faced a major setback in October 1216 when King John's death led to the rebellious barons deserting him in favour of John's nine-year-old son, Henry III of England and the war dragged on. Louis' army was finally beaten at the Battle of Lincoln on 20 May 1217. And, after a fleet assembled by his wife, Blanche of Castile, attempting to bring him French reinforcements was defeated off the coast of Sandwich on 24 August 1217, he was forced to make peace on English terms. He signed the Treaty of Lambeth and surrendered the few remaining castles he held. The effect of the treaty was that Prince Louis agreed he had never been the legitimate king of England. This formalised the end of the civil war and the departure of the French from England.