As of September 1941, the US Navy was waging war against Germany in the Atlantic. This followed numerous violations of the Hague Conventions regarding neutrality, in favor of the Allies. One reason Hitler declared war was that he figured Germany and the US were at war for practical purposes already, and he wanted to be seen as starting the war. Most people in the US wanted to stay out of the war against Germany, but they were becoming increasingly convinced that they wouldn't be able to.
While the US kicked Japanese butt really hard (not to slight our Australian and New Zealand allies), US participation in the war against Germany was not really as vital. By the time US intervention was making a real difference, Germany was on its way down (using 20/20 hindsight here). The Soviet Union was the biggest single contributor to the downfall of Nazi Germany, Britain probably the second. The US role was massive and highly successful, but it was generally to late to be actually decisive.
BTW, does anybody else remember apparent pressure being put on the CIA to say what Bush wanted? Or what they finally put out, which was that Iraq wasn't actually an immediate threat?