Why should the U.K. or Russia or the U.S. be a military superpower but Germany can not?
They can always change their constitution and they have the right to.
WWII and the holocaust was almost 70 years ago, modern Germany is not what it was during Hitler. There is no need to be so paranoid about Germany building up its defense forces. The military to me seams a better place to put its money than a welfare nanny state.
I didn't say they have no right to be a superpower, they have tonnes of right, but they have no need nor wish too. There was a BBC poll some years ago which found Germans to be the least patriotic of all questioned nations (all EU nations I seem to recall). This is a backlash to the Second World War and the Holocaust which they are frankly embarrassed about.
That backlash is important in answering this question, because another part of the backlash was that Germans are generally distrustful of excessive military power, hence the pacifist constitution. Germans detest their countries involvement in Afghanistan, for example.
Basically, the crux of your arguement was that all countries have the right to be a superpower, I agree. However, in any democratic state that will be largely down to the electorate, and the German electorate likes their "nanny" state. Given the choice, they'll vote for welfare over military power. Using your own arguement, the Germans have the right to a welfare state as much as to a superpower status, and they've chosen welfare.
Personally being a Briton I think the British welfare system should be cut back and the armed forces either given proper funding or not sent to so many wars at once, but I would never (EVER) want to get rid of welfare, because I have seen in the US what a lack of decent welfare can do for those without money, and it isn't fair at all. But that's a seperate matter entirely
Sooooo:
Germany has right to superpower status
Germans don't want it (prefer welfare)
So Germany won't become nor wishes to become a superpower
Note:
I have not touched upon the issue of a potential EU superpower, which the French seem to be pushing the Germans towards. Hopefully it'll never happen, and if it does Britain won't have anything to do with it, hopefully... But that won't be a German superpower, that'll be a European one...