I think that Europeans in general are indeed more liberal than Americans, and I think that at least part of that does come from the world wars. Not just from the number of people that died, but also because of the amount of devastation inflicted on infrastructure. Because of American geographical isolation, We've never really had to deal with the aftermath of war. Oh sure, there was the War of 1812 when Washington was burned, and there was the attack on Pearl Harbor, but comparitively speaking we've been unscathed. That leads to an attitude of there being very little risk involved in diplomatic policy, whether it be just throwing our weight around, annoying other nations with our policies, or actually going to war.
There's an acronym used in the US; NIMBY...it means Not In My Backyard. Don't know if Europeans use that or have a similar one. It's usually used when speaking of things that might be needed, but nobody wants them around. Like a new sewage processing center, a city dump, a jail, whatever. Lots of people may agree that such a thing is needed, but nobody wants it built in their hometown or neighborhood. Well similarly, foreign policy politics and war have never been in America's 'backyard'. I think that American attitudes might be a little different if they were.
Many posters stated that Americans feel superior, or are in 'their own world'. I think this can be related to geography as well. We only have 2 neighbors, Canada and Mexico, while the typical European country has 3 or 4 bordering countries, in many cases those borders can be reached in a matter of hours, while in the US reaching a border can take an average person days. From what I understand from speaking to a few friends I have on the net from Germany, Denmark, and the UK, it is also not so rare to have a relative in your family from a different country than yourself, just a few miles away. In my opinion, this leads to more of a feeling of community and comradery across borders than what exists in the US.
Regrettably, I also have to admit that many Americans are just plain stupid...I tried to have a conversation at work once with someone about politics, and had to explain to them who Chirac was in the course of the conversation. *sigh* Even though I consider myself somewhat above average, I only speak a smattering of 2 other languages other than English; Japanese and Spanish. But my Danish friend is fluent in 4, and can get along in 2-3 others passably well. Again, I feel geography has a role in this. Most Canadians speak English (despite their English vs French political schism in recent years), and many Americans look down on Mexicans as just a potential source of illegal immigrants, so nobody much cares about learning other languages in the US.