The three major Allied powers were Great Britain, the United States, and Soviet Union. Their leaders, Winston Churchill (1874–1965), Franklin Roosevelt (1882-1945), and Joseph Stalin (1879–1953), were referred to as the Big Three. They and their military advisors developed the strategy to defeat the Axis countries—though Stalin, for the most part, acted alone on the Soviet front. China, which had been at war with Asian rival Japan since 1937, also joined the Allies. Forty-six other countries became part of the Allied front before the war was over.

Germany invaded Poland on September 1, 1939, and within days Great Britain entered into fighting against Germany. Australia, New Zealand, India, France, South Africa, and Canada also allied with Great Britain, as did Norway, Denmark, Belgium, the Netherlands, and Luxembourg in 1940—all of them under siege by Nazi Germany. Greece entered the war later that year, as did Yugoslavia in the spring of 1941. On June 22, 1941, the Soviet Union entered the war. And in the days after the Japanese bombing of the U.S. naval base at Pearl Harbor, Hawaii, on December 7, 1941, 12 more Allied countries became involved in the war, chief among them, the United States and China. (The others, with the exception of Czechoslovakia, were all Caribbean and Latin American countries: Panama, Costa Rica, the Dominican Republic, Haiti, Nicaragua, El Salvador, Honduras, Cuba, and Guatemala.) The year 1942 saw three more countries join the Allies—Mexico in May, Brazil in August, and Ethiopia in December. In 1943 and 1944—in what were perhaps the darkest days of the war—Iraq, Bolivia, Iran, and Columbia signed on as Allied nations, followed by the tiny country of San Marino (significant since it is situated wholly within the boundaries of Axis power Italy), Colombia, and Liberia. February and March of 1945 saw another wave of nations siding with the Allies: the South American countries of Ecuador, Paraguay, Peru, Chile, Venezuela, Uruguay, and Argentina; along with the Middle Eastern countries of Egypt, Syria, Lebanon, and Saudi Arabia. Mongolia (the Mongolian People’s Republic), in Central Asia, was the last to join the Allies, on August 9, 1945. Of course, the level of support each Allied nation lent to the war effort varied. But it was significant that the list of Allied nations grew longer with each year that the war was fought.