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 is named after the House of Lancaster, the ruling house of the Kingdom of England, to which Henry V belonged.

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. In 1420, the Treaty of Troyes was signed, by which the English king married the French princess Catherine and was made regent of the kingdom and heir to the throne of France. A victory on paper was thus achieved by the English, with their claims now having legal standing. Some of the French nobility refused to recognise the agreement, however, and so military subjugation was still necessary to enforce its provisions. King Henry V and, after his death, his brother John, Duke of Bedford, brought the English to the height of their power in France, with an English king crowned in Paris.

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. Charles VII of France was crowned in Notre-Dame de Reims in 1429, and from then a slow but steady reconquest of English-held French territories ensued. Ultimately the English would be expelled from France and lose all of their continental territories, except the Pale of Calais (which would be re-captured by the French in 1558).

The Battle of Castillon (1453) was the final action of the Hundred Years' War, but France and England remained formally at war until the Treaty of Picquigny in 1475. English, and later British monarchs would continue to nominally claim the French throne until 1801, though they would never again seriously pursue it.