World War II (1939-1945) was a conflict that involved all the world's major countries. It was the most destructive war in history and millions of people were killed. The War began in Europe on September 1, 1939, when Germany invaded Poland. Great Britain and France responded by declaring war on Germany on September 3rd. 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. On December 7, 1941, 360 Japanese aircraft attacked the major U.S. naval base at Pearl Harbor in Hawaii, taking the Americans completely by surprise and claiming the lives of more than 2,300 troops. The attack served to unify American public opinion in favor of entering World War II, and on December 8, Congress declared war on Japan with only one dissenting vote. Germany and the other Axis Powers promptly declared war on the United States.