1. Attack on Pearl Harbor: The immediate trigger for the US entry into World War 2 was the surprise attack on Pearl Harbor by Imperial Japan on December 7, 1941. The assault resulted in the deaths of about 2,400 Americans and severe damage to the US Pacific Fleet based in Hawaii. It was perceived as an act of aggression that violated international law, igniting widespread outrage and a desire for retaliation within the United States.
2. Allied Support: Before the Pearl Harbor attack, the US had provided significant material and financial support to its European allies (Britain, France, and the Soviet Union) involved in fighting Nazi Germany. This assistance came through programs like the Lend-Lease Act passed in March 1941, which allowed the US to lend or lease war supplies (including armaments, food, vehicles, and other resources) to its allies without direct monetary compensation.
3. Japanese Expansion and US Interests: The US also had concerns about the expansionist policies of Japan in the Asia-Pacific region. Japanese military conquests and its control over resource-rich territories threatened American economic interests and the stability of its relationships with allied nations in the area.
When:
The United States officially entered World War 2 on December 8, 1941, the day after the Pearl Harbor attack. President Franklin D. Roosevelt delivered a speech before a joint session of Congress on the same day, famously referring to December 7, 1941, as "a date which will live in infamy." The US Congress subsequently voted to declare war on Japan, leading to its formal entry into World War 2.