Answer:
The USA entered the war after the surprise Japanese attack on the Navy facilities at Pearl Harbor, Haiwaii, occurred on December 7, 1941. President Franklin Delano Roosevelt asked Congress to hold a special session and Congress declared war on the Japanese Empire. This is the direct reason for the USA to join Allies in WWII. However, it was expected that sooner or later the US would enter the conflict on the Allied side against the Axis Powers; though formally neutral, the American government had been providing weapons and supplies to Great Britain.
Step-by-step explanation: