The US joined World War 2 after the Japanese attack on Pearl Harbor on December 7, 1941. This surprise attack crippled the US Pacific fleet and caused widespread outrage among the American public. In the days following the attack, the US declared war on Japan and entered World War 2.

There were a few key reasons why Japan attacked Pearl Harbor and the US eventually joined the war:

1. The US was supporting the Allied powers of UK and Soviet Union with aid and supplies. This angered the Axis powers of Germany, Italy and Japan.

2. The US had placed embargoes and sanctions on Japan, including cutting off oil supply. This threatened Japan's military ambitions and their effort to build their empire in Asia. The attack on Pearl Harbor was a preemptive strike by Japan to weaken the US fleet.

3. The overall aggression and expansion by the Axis powers (Germany, Italy, Japan) threatened US interests and allies. By 1941, the war in Europe had been raging for 2 years and Japan had been invading other countries in Asia. After Pearl Harbor, the US decided to enter the war fully to fight the Axis alliance.

So in summary, the key reasons for US entry into WW2 were: the Pearl Harbor attack, US support for Allied powers, and the aggression/expansion by the Axis alliance (Germany, Italy, Japan) which threatened global stability. The Pearl Harbor attack was the final catalyst that led to the US declaration of war on Japan and entry into World War 2.
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