During the WWII, Japan was planning on setting up a perimeter in the Central Pacific, thus giving it complete access to the resources in South Asia. In an attempt to completely prevent the US pacific fleet from interfering, it had plans to neutralize it early on as well. The main reason why the US joined the War was because Japan attacked it's entire fleet stationed in Pearl Harbor, causing great damages to the US naval forces.
Among that there was a lot of pressure from the 'allies' if Europe would have been taken the US would have found itself in the middle anyway the events like the USS Mesopotamia and Pearl Harbor were just to get the US involved, either way it was inevitable. But there is also the fact that Hitler had his sites set on the Americas in the sense that he was going to let the Japanese have there way with the US and Canada they had plans to come in through the Arctic and S. America. But that was easily thwarted when we entered the war earlier then expected, in fact I am sure Hitler sh*t him self when he got the news....
Well many things were going on. Japan bombed pearl harbor and so we joined ww2