England and France have fought myriad wars throughout the centuries, so it is difficult to know which specific conflict you are referring to. There have been many Anglo-French wars. But the two nations have not—except for a brief conflict between England and Vichy France during World War II—fought a war since the defeat of Napoleon at Waterloo in 1815. England and France went to war together as allies during World War I and World War II. Today, they remain allies as members of the North Atlantic Treaty Organization, and they are connected geographically by the Channel Tunnel.
The long history of warfare between the English and the French stems from their geographical proximity and intermingled royal lines. In 1066, the Normans—who came from a French province—conquered England and this event partially unified the two nations. Thereafter, the kings of England claimed to be the rightful kings of France, too; the Hundred Years' War (1337–1453) was fought because of this dispute over the French crown. France finally won that war, and England lost almost all of its territory in modern-day France.
Later, the two nations fought over colonies in North America and India. England prevailed in those contests, but France revenged its defeat when it helped America achieve independence from England in 1783.
The Napoleonic Wars (1803–1815) were a monumental struggle. Napoleon had tried to invade England, but he did not have a strong enough navy. England, with the help of other nations, finally defeated Napoleon at Waterloo.