It's interesting and being i just finished my AP History course last semester it's still slightly fresh in my mind! My teacher made it a point that Yes the United States was in jeopardy of attack from the Axis Powers. Now although it wasn't as immediate a threat to the U.S. as it was say Britain. You can use events like Pearl Harbor, the Japs attack PH mostly for the reason to eliminate the American Fleet and make it an easy route in. Although the U.S. was not on the immediate TOP of the invasion list i believe that given the opportunity they would not have hesitated. This is even a reason why the American public supported the US's involvement in the War. It was the first time America had been "attacked" on home soil probably since the Revolutionary War! All in all it sounds like you have a stubborn teacher but, yeah i think if America hadn't invaded Europe and Stopped the German advance, as well as the Japanese aggression the U.S. would be in a bad spot. If we hadn't stopped the German aggression in Europe there would have been no route for America to fight Germany. If Great Britain fell to Germany where would the U.S. deploy troops? This brings about the fact that Germany would be able to build, and possibly cause the U.S. to fight a two front war (what Germany ultimately fell from)!