America’s foreign policy regarding entering WWII was forged in the Great Depression and WWI. Both were traumatic events in the history of the US and caused the government to become slightly isolationist as a result. However, the US did eventually support the British war effort by supplying arms in 1940 in ‘cash and carry’ deals.
After the attack on Pearl Harbor, the US officially declared war on the Axis forces and became a major player in the conflict. By the end of the war, the US has suffered 407,000 military losses and dropped the first atomic bomb on Japan, bringing and end to WWII and sewing the seeds of the Cold War.