Why did the US commit itself both to the pacific and Europe in World War 2?

Japan attacked the US in the Pacific so we had to fight them there. Germany declared war on the US on December 11, 1941; so we had to fight them too. Germany declared war on the US hoping Japan would open a "second front" on the Soviet Union, to relieve the Germans fighting on the "Russian Front." Japan had already fought the Russians twice, once in 1904-1905, then again in 1939; The Japanese were weary of fighting the Russians, and didn't want to fight them again, unless they absolutely had too.