Answer: The Attack on Pearl Harbor
Explanation: At first the United States was strongly believing in a Isolationist view on the problem. At first the problems were just percieved as a general dispute between nations, nothing that should concern the US to step in or say anything. There was no warranted action at the current time that World War 2 was starting up. The Japanese sent a message to the United States though, through sneak attacking one of their ships in Pearl Harbor. The Bombing of Pearl Harbor left many dead and a ship in destruction. After this had occur, many wished to hit Japan back and even harder, which started the War effort on the United States side.