the matchup in weapons and numbers sounds pretty even when you take into acoount America's nasty secret weapons (stealth bombers, Aegis missiles, other nasty shit like that) but the US also has 4 mothballed carriers and a shitload of mothballed planes.

Europe would not just one day say "look US, we don't like you so we're declaring war", there would be a long term development of events, plenty of time to get good quality ships and planes out of mothballs and train enough pilots/sailors.


but realistically speaking I think there need to be more specifics before we can even begin to argue. For example, the idea of all of Europe agreeing to attack the US is not very likely (even the Allies in WWII were hard pressed to stay friendly to each other, if not for FDR we would be screwed).
It is much more logical to say that the EU would be involved in a future war with the US. From reading material on a politics forum I have learned that the new EU constitution does everything but actually say that individual members have no sovereignty, so EU is likely to be the next big bad dictator state, like Nazi Germany and the USSR.

By some extension of the imagination I can see the US and EU getting in a trade war in the future (a sort of Euro vs. Dollar thing) which could lead to a shooting war. It is rather hard to determine who the UK, Russia, Commonwealth states, and Asian states would side with. After the war with Iraq it seems that the US is trying to be more friendly with Russia, relations with Germany and France are still rarther cold.