World War II began in Europe on September 1, 1939, when Germany invaded Poland. Great Britain and France responded by declaring war on Germany on September 3. The war between the U.S.S.R. and Germany began on June 22, 1941, with Operation Barbarossa, the German invasion of the Soviet Union. The war in the Pacific began on December 7/8, 1941, when Japan attacked the American naval base at Pearl Harbor and other American, Dutch, and British military installations throughout Asia.