If Heidelberg's administration is anything like Darmstadt's, the buildings will be abandoned and empty but well guarded till they fall into a state of dilapidation.
At least by now none of the well maintained and repaired buildings there have been turned into student homes.
The US has the ability to take control of any country within 24 to 48 hours. Every year we draw up invasion plans for everyone else in the world. The CIA World Factbook [1] is the unclassified by-product of this effort if you are interested at poking into the details.
As with any other country with a large standing army (Germany sits somewhere between South Korea and France in terms of capabilities), it would probably begin with large scale air strikes with stealth bombers against military targets. Within the first 4 hours, most militarized airfields and radar installations would be destroyed. The ground forces would invade from probably Italy and Poland, both highly US dependent states, assuming that France and the UK would stay neutral in such an attack. You'd probably see some guerrilla warfare because the Germans are strong-willed, but the vast majority of the population is westernized enough to not put up a real fight. But none of this would happen until the US has at least 2 carriers in the Baltic Sea, so Germany would definitely have advance warning, and hopefully the foresight to negotiate.
Germany has no nuclear deterrent, as it simply hosts US missiles by way of NATO [2].
That's all very nice, but while the US seems to be able to invade countries at will, they don't seem capable of occupying even the weakest of countries - Iraq and Afghanistan being cases in point.
