Let me reiterate my defense of the Germans. As Mzungu touched upon, there was a large stretch of time spanning the Late Middle Ages and Early Modern periods in which Germany (well, the HRE) was nowhere near the most aggressive nation. It's true that a primary reason they weren't so aggressive was because they were fighting amongst themselves most of the time, but something has to be said for having the largest concentration of armored knights in Europe (IIRC), but rarely deploying them for conquest. After all, both the states and Emperor made up the HRE government, and because a large component of it (the states) were invested in expanding within, rather than outside, of the borders, it's also possible to say that a large component of the HRE government was disinterested in being aggressive outside its borders. So, I'm not so eager to label Germany as the most aggressive nation ever. But all said, with the World Wars and all, they're certainly up there.
Well, you're just emphasising what I've said. When Germans were not conquering other nations, they were busy with slaughtering themselves. The Thirty Years War in which two thirds of the German population got killed is just one example. And if the HRE had the largest density of knights, even though they were not driving them outside of their borders (what they actually did btw, as already mentioned in Italy and Eastern Europe) it was surely not for peaceful reasons. Even from 1949 till 1990 German tanks were pointing at each other.
So after 1945 we Germans were well aware of the problem that on a long range constant of two millennia, AND in absolute brutality as in the Third Reich, Germans HAVE been the most aggressive nation in world history. If you pick every single incident of crimes other nations have done (Conquisition, slave trade, Gulag, Apartheit etc...), there is NOTHING that couldn't again have been topped by some German crime.
In fact it is a problem that puts Germans into an identity crisis still today, as it would suggest that every feature of German culture is part of an ungainly inhuman machinery. It became very unfashionable, especially among young people, to practice anything which could be connected with being 'German', inclusivly sometimes music, cuisine or dressing. The result was almost some sort of complete cultural tabula rasa after 1945, in East Germany even from 1990 on. I would claim, this makes even the US far off richer in tradition and culture than Germany.
We don't have a national holyday in Germany that is in fact 'celebrated', besides the 3. of October (day of unification), which is actually just a day off, nothing more.
We don't have a single song in Germany that EVERY German, young till old, could sing. Including the national anthem. And perhaps besides this song:
http://www.youtube.com/watch?v=nNT0fQnZBOw
I don't know any German about my age who could cook things like Sauerkraut or other traditional German meal. German cuisine is usually frowned upon and only accepted in the case of cheap fast food (like Bratwurst, Schnitzel etc..)
Unlike France for example, there is no serious movement which tries to keep out foreign words out of the German language.
I could continue with this list. It is a fact that German culture has been very aggressive for two millennia, and this has still impact on German thinking today.