>USA on the other hand is an "engineered" country, people got there relatively recently and said "Ok, that's it, let's do things this way". Of course you will see more homogeneity and agreement. And nationalism actually helps keep stuff together instead of dividing people.
Go study up on the American civil war. It was an extremely brutal conflict, and you see lingering resentment today, in the 'red states' and the 'blue states' - hell, I still make (offensive) jokes about how "we should have let them leave when we had the chance" - I mean, I don't know how it compares to the Germany vs. France thing, but even today, there is a pretty significant north/south divide in US politics (and urban/rural divide.) From my understanding of things, we (the northern, industrial states) conquered the southern, agricultural states through force of arms, (well, through force of superior production) - and we've kept them in the union since through a generous application of money. (It's seen as funny, that the "red states" generally speaking, consume more federal tax dollars than they give back, and yet those are the people who make a lot of noise about cutting the size of the federal government. The "blue states" are the opposite. The way I see it, we just figure it's cheaper to buy them bread and circuses than to fight 'em again, while they are frustrated by their powerless position. From what I read, (and culturally, I think) we think of the south as producing brave, but poorly equipped soldiers. (and often we think of the southern leadership as having more guts than brains, that, or just having generally less concern for the lives of their soldiers. The south, at least by Northerners, is perceived as having a lot more respect for it's aristocrats, and less respect for it's peasants, vs "the only color that matters is green" north, which tends to style itself as a meritocracy.) The turning point of the war is often considered to be 'Pickett's charge' - A very brave, but arguably, considering the weapons the union had in the area, the space they needed to cross, and the range and effectiveness of the Minié ball, very stupid military action.)
My point here is that being more homogeneous (and we are... there are big cultural differences, but we all mostly speak the same language) does not save you from war. Civil war can be just as bloody and brutal as wars of conquest between different countries. When you get down to it, brothers will slaughter oneanother just as readily as strangers who don't speak the same language.
Perhaps the more powerful EU countries should get used to giving handouts to the poor EU countries in order to keep things quiet; that is how I think things work here