The Lancastrian War was the third and final phase of the Anglo-French Hundred Years' War. It lasted from 1415, when King Henry V of England invaded Normandy, to 1453, when the English lost Bordeaux. It followed a long period of peace from the end of the Caroline War in 1389. The phase was named after the House of Lancaster, the ruling house of the Kingdom of England, to which Henry V belonged.

Henry V of England asserted a claim of inheritance through the female line, with female agency and inheritance recognised in English law but prohibited in France by the Salic law of the Salian Franks. The first half of this phase of the war was dominated by the Kingdom of England. Initial English successes, notably at the famous Battle of Agincourt, coupled with divisions among the French ruling class, allowed the English to gain control of large parts of France.

The second half of this phase of the war was dominated by the Kingdom of France. French forces counterattacked, inspired by Joan of Arc, La Hire and the Count of Dunois, and aided by the English loss of its main allies, the Dukes of Burgundy and Brittany.