The Hundred Years' War was a series of conflicts waged from 1337 to 1453 by the House of Plantagenet, rulers of the Kingdom of England, against the House of Valois, rulers of the Kingdom of France, over the succession to the French throne. Each side drew many allies into the war. It was one of the most notable conflicts of the Middle Ages, in which five generations of kings from two rival dynasties fought for the throne of the largest kingdom in Western Europe. The war marked both the height of chivalry and its subsequent decline, and the development of strong national identities in both countries.
Historians commonly divide the war into three phases separated by truces: the Edwardian War (1337–1360); the Caroline War (1369–1389); and the Lancastrian War (1415–1453). Local conflicts in neighbouring areas, which were contemporarily related to the war, including the War of the Breton Succession (1341–1364), the Castilian Civil War (1366–1369), the War of the Two Peters (1356–1369) in Aragon, and the 1383–1385 succession crisis in Portugal, were availed by the parties to advance their agendas. Later historians adopted the term "Hundred Years' War" as a historiography periodization to encompass all of these events, thus constructing the longest military conflict in European history.
The war owes its historical significance to multiple factors. By its end, feudal armies had been largely replaced by professional troops, and aristocratic dominance had yielded to a democratisation of the manpower and weapons of armies. Although primarily a dynastic conflict, the war gave impetus to ideas of French and English nationalism. The wider introduction of weapons and tactics supplanted the feudal armies where heavy cavalry had dominated, and artillery became important. The war precipitated the creation of the first standing armies in Western Europe since the time of the Western Roman Empire and thus helping to change their role in warfare. With respect to the belligerents, in France, civil wars, deadly epidemics, famines, and bandit free-companies of mercenaries reduced the population drastically. English political forces over time came to oppose the costly venture. The dissatisfaction of English nobles, resulting from the loss of their continental landholdings, became a factor leading to the civil wars known as the Wars of the Roses (1455–1487).