In 1939, America decided it would stay out of WWII, as it was a European war. Eventually America decided to intervene.
After the war started and it looked like America's allies would have a difficult time winning, America decided to start selling weapons to Britain, but only if they came to us and payed in cash.
America also sold them destroyers if America could gain the right to build military bases around the Atlantic Ocean.
America eventually declared the western hemisphere neutral, and had ships patrol near the boundary of the western hemisphere and tell Britain the location of any submarines encountered.
After America stopped sending large amounts of oil and iron to japan, they attacked Pearl Harbor and America quickly joined the war.
Throughout the war America's economy kept climbing out of the Great Depression. This is due to the jobs that mass production created. Even females were allowed to start working in places that were male only before, such as factories for gun and ammo production.