Despite the Japanese invasion of China in 1937, most historians date the start of the Second World War as September 1, 1939, the day that German forces attacked Poland. Although Polish resistance was quickly overcome, treaty obligations brought Britain and France into the fray, and the war for Europe began in earnest.

Strong isolationist sentiments among much of its populace kept the United States out of the conflict until the Japanese bombed American naval forces anchored at Pearl Harbor in Hawaii. The surprise attack, which inflicted devastating losses on the U.S. fleet, occurred on December 7, 1941. The next day, President Franklin Roosevelt asked for, and received, a Senate declaration of war on Japan. Two days later, Germany and Italy, which were bound to Japan in a mutual-defense treaty, declared war on the United States. The fighting continued until August, 1945, when Japan (the last Axis belligerent left) surrendered...