This was due to several reasons, including a strong isolationist sentiment among the American public, the devastating effects of the Great Depression, and a belief that the United States could best serve the world by staying out of the conflict.
However, the United States gradually became more involved in the war as it provided increasing support to the Allies. In 1941, the United States began providing lend-lease aid to the Allies and began to patrol the Atlantic Ocean to protect Allied shipping.
After the Japanese bombing of Pearl Harbor in December 1941, the United States formally
entered the war on the side of the Allies.