This discussion is a bit like my dad is stronger than your dad... but kind of fun.

I believe that you are right that the world together could not invade the US... but if there were a war lasting 10-20 years I guess the scales over time would tip enough to make it possible. Winning a war involves lots of aspects. I am sure that the rest of the world would have so much more resources to put into creating whatever was needed where the US would run low on a few resources. Maybe the right inventions would take more than 20 years but the world wars have shown how fast inventions can be made when you really really really want them to.

-Peter