America was known for its isolationist attitude towards the problems in Europe both during economic depression before the war, which was potentially caused the expansion of Nazism and the possibility of war, as well as the issue of World War II itself. It changed after the Japanese attack on Perl Harbor in 1941, when the entire American public was shocked and outraged, after which America declared the war on Japan.
America mainly stayed neutral during WWII. When Japanese forces bombed Pearl Harbor, Roosevelt felt it appropriate to declare war on Japan, thus entering the war.
