Double jeopardy is a concept in most legal systems around the world, not just the US.  We inherited it from the UK, as did the rest of the Commonwealth legal systems, and many other legal systems came up with the concept on their own.  Pretty standard in non-crappy legal systems.

 

But double jeopardy is a concept only as to any given sovereignty.  US law prohibits a state from prosecuting twice for the same offense, but does not prohibit a state for prosecuting for a crime, even if the defendant was already tried on the same offense in another country.

 

In fact, it is pretty common.  For really bad people, it is not unusual, for instance, to serve 20 years for murder in country A, and then be extradited to country B, where they are promptly tried and convicted again for the exact same murder.

 

In the US, we can even do one better:  the Federal government and the various states are seperate sovereign entities.  Some crimes, like murder and bank robbery, are both state and federal crimes.  The law allows state and federal governments both to try a man for the same crime.  As a matter of federal policy, we don't do that, but the legal theory is there.

 

On point - the old nazy guys are a very difficult matter.  Difficult because of the time that has lapsed, and difficult because of the simply tumultuousness of the times.  I have a fair amount of sympathy for the Nuremburg defense.  There are matters of degree to all things, of course, but virtually everything that happens in national conflict would be a "crime" under civilian law.  IMO these things are more political than anything else, and we mostly make up the rules as we go along, to satisfy our sense of moral indignation.