The Allies were the countries united in an alliance in World War II (1939–45) to fight the Axis countries of Germany, Japan, and Italy. The countries that first declared war on Germany after its invasion of Poland in 1939 were the founding Allied forces. They were Poland, Great Britain, and France.
Many countries joined the Allied efforts over the course of the war. Twenty-six countries signed the Declaration by the United Nations on January 1, 1942, uniting them in the Allied cause. More nations would sign later.
The leading efforts of Great Britain, the Soviet Union, and the United States in the second half of the war earned them the distinction as “the Big Three.” Prime Minister Winston Churchill (1874–1965) of Great Britain, Premier Joseph Stalin (1879–1953) of the Soviet Union, and President Franklin D. Roosevelt (1882–1945; served 1933–45) of the United States coordinated the military efforts of the Allies across the
world. China and France, the latter when not occupied by Germany and its Nazi dictator, Adolf Hitler (1889–1945), played important leading roles as well.
The Allies managed to defeat the Axis powers even though their resources were stretched around the world. After the capture of Italy's dictator, Benito Mussolini (1883–1945), Italy switched sides to fight with the Allies in September 1943. The battles across Europe ended with the unconditional surrender of Germany on May 7, 1945. Battles around the Pacific Ocean against Japan continued until the United States used the atomic bomb for the first time to end the war. After suffering the devastation of the cities Hiroshima and Nagasaki, Japan agreed to surrender on August 10, 1945.