I think that the reason Germans collectively dismiss military action is that WWII stigmatized Germany. The German people still live in the memory of Adolph Hitler’s aggression. Hitler’s fanatical idealism is still a haunting factor in the psyche of the German people. Every country is more cognizant of their history in a more personal way.
Americans have never experienced life under dictatorship as Germany has.
There is the exception of some far left whacko’s that think Bush a Nazi and an Imperialist, but that doesn’t count. I think that the path of war for any reason reminds the German people of Adolph Hitler’s Germany. The memory of Adolph Hitler still suppresses the German populace in giving a green light for war for any reason other than defending Germany’s borders from hostilities.

It's neither A nor B or C.

World War I had an effect between the two world wars but not afterwards. Few people in Germany believe that Germany was morally wrong to fight World War I.

The culturally most militaristic part of Germany is Brandenburg, not Prussia per se. Prussia was originally one of the Baltic countries but the centre of "Prussia, the German state" was the Prussian province of Brandenburg, which is now the state of Brandenburg, which took over representing the country of Prussia. "Prussia" as we know it military-wise is quite intact, although not militaristic any more, but not because the hinterland in the Baltics and Pommerania was lost. (Incidentally, my own home state of Berlin is an island within the state of Brandenburg.)

As for C, I tend to believe that the more militaristic people were also those who were in command during World War II and ordered others to fight rather than risk their own lifes.

German pacifism, I think, is simply a result of the cold war. With the Russians nearby Germany had only two options, one of them was to be quiet and wait it out.

(Although Anthony's point is also valid.)

You have to realize this is a top down view on the most warlike place on earth, the birthplace of armageddon. I can't think of any continent halfway on the other side of the earth being as bad, as warlike, as intrinsicly carnal, brooding, and fey, at least in the latter centuries. Yet by the same, the birthplace of science, mind over man.

The Mongols exterminated a far greater percentage of the earth's population than Hitler or Stalin.

Germany is, in my opionion, mostly acificistical simply because they have had a serious taste of what war can do to a nation, a people , their reputation, their self-esteem and not to mention how many actually got killed. Most, if not all, germans today have a father or grand-father who was killed in the war. Most adults remember the hard times rebuilding Germany after the war, they all learn in school what the war did to them etc etc.
So it is, I believe, a matter of learning-by-mistake... They fear war, quite simply.

And the referece to the mongols is, IMO, not relevant at all. Why ? Simple: They lived and raided a long time ago, little is really known and depicted (pictures/movies/etc) from the time. And how many lives today that has a forebearer that actually died by the hands of the Mongols ? few I believe, and close to nobody in the Western world...

For me the vast amount of information and first/second hand experience with the war and its effect on Germany is the single most important reason for them to be so ill at ease about war.