Edit: This is for Little Skittle's 'rebuttal'.
Remind me how many Russians the Nazis had left to kill?
Well, not that America has ever been a true democracy, but even in democratic countries, the decisions are made by the few. Or would you have me believe that before getting involved in a war the US government consults it's people? Man, the government doesn't even ask 5% of the population enver mind listen to it!
In politics there is a little thing they like to do - not always that successfully mind you - & it's called 'thinking ahead'. Oh yeah, would you believe that your government does not make decisions on a whim? As groundbreaking as this may be to you, your government will have weighed up their options - just like they did in WWI when they heard the money cow sounding - & make a decision based on the conclusion of their evaluation.
Do you think America were happy to go at it with the Japanese Empire alone? Especially when they knew that the Allies - with bases in Asia - could help them? Also, when you think of how the Japanese just blatantly slapped America in front of the whole world, exposing the US as slow & somewhat weak? Then of course, Pearl Harbour left the US government needing to take action, in fear that the Japanese - or other nations, including Nazi Germany who remember were taking over territories in Europe & Africa already, buoyed by the Japanese success - would attempt another strike?
Take into consideration that America employed a stern policy of isolationism. The only reason that they would get involved in foreign affairs is for money - which they were even reluctant to go to war for following WWI - or if they were feeling threatened. The answer is the latter btw.