False. The Nazi's never controlled Europe. Spain was neutral, she hated the Nazi's but she had just come out of a civil war, Fransico Franco was not interested one bit to plunge into another war. Switzerland was neutral, again, NOT UNDER NAZI CONTROL. Italy was an ally, not really under Nazi control. So your statement entire "EUROPE has been conquered" is false.
Several things. Isolationism means that one country avoids getting involved in international matters (and that includes trade). What the U.S was doing was NONINTERVENTIONISM (a policy that means you dont get involved in things that are not your business but you still trade with the world.
The U.S saw Europe as constantly feuding, it did not want to get involved in that. Not on our soil, not our problem. Same mentality occurred during World War One. Why should we get involved in European matters, the war was a royal rivalry anyway, so until the unrestricted submarine warfare occurred, the Americans were perfectly happy to be neutral.
Since when does the U.S have the right to be the policeman of the world?
The U.S was in a depression anyway so.....there you go. It was none of their business and they were in a depression. In fact, had the Japanese not be allied with Hitler, the Americans would have only been fighting in the pacific.