Question

What did Japan do that caused the American to get directly involved in World War II

Answer 1

Japan surprised attacked Pearl Harbor, destroying most of the US's Pacific Fleet (but missing the carriers that were out on a manuovre), even while the Japanese diplomat was talking of peace. 

Later on, Japan also took over US colonies, such as Wake Island, Phillipines, etc.

hope this helps

Answer 2

The surprise attack on Pearl Harbor which American saw as a pure act of aggression and declared war on japan as a result