The main reason the US eventually joined WWII was because Japan attacked Pearl Harbor on December 7, 1941. This event prompted the US to declare war on Japan the following day. Additionally, the US had increasingly been involved in supporting Allied countries through programs like the Lend-Lease Act and the Destroyers for Bases Agreement. The U-boat attacks on American ships in the Atlantic and the discovery of the Zimmerman note, which revealed Germany's plan to ally with Mexico against the US, also contributed to America's decision to enter the war.
