Dec 7, '41.
The u.s. DID get involved.
yes they were
No
yes
World War 2
the US by FAR.
December 8, 1941 was the date of the US declaration of war
The United States became involved in the World War II about March 1941.
The US was involved for 6 years from 1941-1945
Japan
the USSR and the US
The US contributed some 16 million personnel to the Second World War. I'd say that would indicate they did get involved.