How do I take advantage of the Japanese Anti-American offense?
One thing that you could do would be attacking anything in the Japanese sea zone with the American fleet.  The British would then advance on Japanese controlled Asia and with the Americans keeping the Japanese from building ships, it will make it a lot easier as the game continues.  The US will still need to build a slightly bigger navy in order to destroy the rest of the Japanese.  Try not to break up your navy into several smaller armadas.  Just hold off the Germans the your British in Europe and the Russians.  Once Japan is relatively helpless, you can take all but one ship from the Pacific and then, if you can keep Japan at bay, you can start to creep into Europe to help your allies.  Hope this helps!