The United States had no choice but to join World War II. Prior to World War II, the United States was officially neutral. However, we were providing aid to Great Britain in various ways. We were giving Britain destroyers as a result of the Destroyers for Bases program. We were loaning and leasing supplies to Great Britain as part of the Lend-Lease Act. We were monitoring the western Atlantic in order to let the British know where the German submarines were located.
We also stopped doing things that would help Japan accomplish its aggressive actions in Asia and in the Pacific. We stopped selling them oil and scrap metal that they could use for their military operations. We froze their assets in our banks.
All of these actions were done while we were neutral. However, when Japan bombed Pearl Harbor on December 7, 1941, we had no choice but to go to war against Japan. We had to respond after they bombed our military and killed many soldiers as well as destroyed much of our military and other property. There was no real viable option except to declare war on Japan.