I am an American and have lived in Europe for the last decade (please excuse any grammar mistakes I made for this reason). I would like to say that our views on war began to fundamentally diverge from those of Europe (specifically, continental Western Europe) after WWII, in my opinion. It is fairly easy to sum up:
After WWII, many turned in silence to the ruble and dust that was once their homes, towns, and cities and said to themselves (I presume this was particularly done in Germany) "Never again! Never again will we let anyone pull the wool over our eyes in such a way as to dupe us into going to war." Some say there is hardly a square meter of countryside where noone was killed in one war or the other over the centuries. Not only that, they also began to question the role of religion in society as the churches, for the most part, did not have the cohones to stand up and and put their monstrous foot into anyone's door.
In America, however, the atmosphere must have been different (even though I wasn't born then). We were the "heroes and liberators" (and rightly so- though many in Great Britian, for ex., fairly dispise the fact that America so often conducts itself - insert about every other Hollywood movie here- as if it won the war on its own without help and sacrifices of others. Who was it that invented the radar / broke German codes / lost most civilians / lost most soldiers in battle, again? Note that at least two countries are being referred to here.).
We (along with the Russians and others: http://www.merkki.com/russians.htm ) freed the concentration camps- it should not be forgotten, however, that these were also where hundreds of thousands of Sinti and Roma -otherwise known as "Gypsies" http://www.remember.org/witness/wit.vic.gyp.html -, homosexuals, POW's, and political dissenters of all sorts and types were to perish... in addition to the roughly 6 million Jews http://auschwitz.dk/Auschwitz.htm , who formed the main block of all those mercilessly slaughtered) and that was most definitely a good thing. That said, it meant that at about the same time others were saying "Never again will anyone con us into a war...!"
we - our mainland soil not having been directly attacked for the duration of WWII - were saying "Just look at what good we can bring to the world when we have a powerful military!"
I can't really speak for France as I have not lived there, but I can almost guarantee that this is the root cause of most pacifism in Germany, today (and is surely why so many here opposed the 2nd Iraq war). I believe many have developed a keen sense of smell whenever something turns foul. For ex., when a major world leader describes the clearly "offensive" act of a first strike on another country as a matter of its own self-defense (which I believe is exactly what Adolf the Terrible did). It is arguable, perhaps, whether our invasion of Afganistan was technically a first or second strike (I believe most of the world gave us the benefit of the doubt, here. Did you know that I, just an ordinary American living in Germany, actually got a letter from a high-ranking German official directly after 9/11 stating that "We in the German government want you to know of our utmost shock, our heartfelt sympathies; and that we stand beside our American brethren in this time great dispair.). This argument does not hold up in the case of Iraq.
Many Americans are under the false impression that they live in the free-est country in the world. They also think the entire rest of the world looks up to them to the point of severely straining their necks (either that or their insides swell with the hate caused by their inherent jeolousy of our freeedoms). This is simply not true. I feel it is very important for my fellow countrymen and women in America to understand this: people over here mostly admire us -including Texans!- because we're just so "laid back" and easy to get to know; because we say "You can call me Jim" and skip a lot of the formalities that they have over here (though I once overheard two Germans in a restaurant speaking of this in a deragotory sort of way. One said "So, you can call me Jim. Sure, I can fire you anytime I want, but you can still call me Jim.") and last but not least because of our creativity; our diversity; and our openmindedness.
Nationalism, in my opinion, rears two main "heads" (one head, naturally, being uglier than the other). One of these heads is "Our country is the 'over everthing' (in some cases simply because the person is unaware of other places on Earth and "because it is where I was born and brought up" as the saying goes) and we can and will kick everyone else's ass just because that is what real men do. The other is "I take pride in my country; in what it has to offer its own citizens and it what it has to offer the rest of the world."
The first form of nationalism is unacceptable in many places here in Europe. An experiment: Go out and wave a standard German flag around on any German street during daylight hours (and make sure that everyone around you understands that you are not waving it because of the national football team or "soccer" as it is refered to in some circles). I can almost assure you that some old lady will eventually come up and beat you over the head with her umbrella or people will start whispering amongst themselves "Look! There's some Nazis over there....".
After the end of WWII, the Germans famously removed the line "Deutschland, Deutschland über alles, über alles in der Welt" ("Germany, Germany, above everything- above everything in the world") from their national anthem. Something similar would happen if you said "Only buy German!" (Can you imagine this happening in America?). When I first came to Europe, I was once taken aback when a German with a doctorate degree in English asked me off the cuff "Did you know that your country has one of the most violently oriented national anthems of all countries in the world? It goes '... And the rockets' red glare, the bombs bursting in air, Gave proof thro' the night that our flag was still there.' In other words, so much of it is about glory and battle and war. http://www.infoplease.com/ipa/A0194015.html (Officially adopted by congress in 1931, this is not the entire text, which goes into even more gorey detail).
To go back to what I said before: many Americans are under the false impression that they live in the free-est country in the world. They believe "that everyone and that person's uncle should have the right to own firearms" was nothing less than what our nation's forefathers had in mind immediately before they fell asleep each night and of course when they did fall asleep, they mostly dreamt about kicking some other nation's ass instead of living in peace along with their neighbors; having finally escaped the politcal amd religious tyrannies which one haunted them in their countries of origin. I wonder if even Quakers would be allowed to form, without massive resistance from various religious groups, under present day circumstances if they hadn't already existed for so long in the past.