Why did the US enter the Second World War?

The United States entered the second World War following the unprovoked attack on Pearl Harbor by Japan on 7 December 1941. 

Up until that time the US was neutral at the start of the war with its Congress being divided over helping Europe against Hitler or not interfering in another European War

For further reading check the following articles:

• World War I
• World War II
• Causes of World War I