As the war was starting to break out in Europe the President of the United States felt the U.S. needed to be involved. Even though the Neutrality Act of 1937 was enacted which basically limited the U.S. from involvement of any future wars, President Roosevelt wanted us in and Congress agreed with him. Congress even went as far as to amend the Neutrality Act so that the U.S. could be able to help the French and the British by providing weapons and ammunition. During all these amendments and trades that was happening with the United States and other allied country's, China which was one of the United States' allies as well was viciously attacked by Japan. When President Roosevelt heard about the attack Japan made towards one of the regions in China he decided to cut off all trades and deals with Japan immediately.
Japan was not to happy with the United States at that point, especially knowing that the resources the United States provided were very crucial to the Japanese soldiers. So because of the shut down of all trades the Japanese saw the United States as not only an enemy but the biggest threat to them as well. The United States then held a meeting with the British on how the United States can help in any way during the course of this war, both agreed to focus on Germany and finding ways to defeat them. While the United States was helping out the British to defeat Germany, the Japanese came out of nowhere soon after to attack the United States in Hawaii. This was known as the attack on Pearl Harbor. After the attack happened the following day the United States declared war on Japan.