This is simply not true. Prosecutions for war crimes have happened both for winners and losers of wars. The truth, to be more exact, is that the losers are overwhelmingly more prosecuted than the winners (by the winners), but the winners now have the option to be brought before the ICC in the Hague.
Of course the chances of countries being signatory to (and having ratified) that particular treaty are roughly proportional to the inverse of their activities regarding such crimes in the recent past.
"U.S. President George Bush today signed into law the American Servicemembers Protection Act of 2002, which is intended to intimidate countries that ratify the treaty for the International Criminal Court (ICC). The new law authorizes the use of military force to liberate any American or citizen of a U.S.-allied country being held by the court, which is located in The Hague. This provision, dubbed the "Hague invasion clause," has caused a strong reaction from U.S. allies around the world, particularly in the Netherlands.
In addition, the law provides for the withdrawal of U.S. military assistance from countries ratifying the ICC treaty, and restricts U.S. participation in United Nations peacekeeping unless the United States obtains immunity from prosecution."
If anybody doesn't, have you ever actually looked at the laws enforced by the ICC and how the judges and prosecutors are appointed? And who has the authority to change those laws? Just because you call something the International Criminal Court doesn't mean that it isn't a ridiculous Kangaroo court.
Without going into a detailed review, the laws all look so vague that any participant in any kind of conflict could be changed with a huge pile of crimes, depending on the whim of the judges and prosecutors. Most sane legal systems require that any law be defined strictly enough that an individual can always be sure whether an act he is planning to commit does or does not violate the law, and I think this body of laws is far, far away from that point.
As for the governing bodies, it seems the primary authority is a body in which every member nation has a single representative and a single vote (the Assembly of States Parties). Uh yeah, I bet about 10 seconds after this court was given any real power, the dictatorships of the world would horse-trade enough votes to change anybody who was politically unpopular with them with a huge pile of crimes whose definition is so vague that it's impossible to know whether you've committed them or to defend against the accusation of them.
The U.S. opposes the ICC on principle, and there are Constitutional issues as well. The ICC is an anti-democratic institution, and participating in the ICC necessitates ceding part of your sovereignty to a bunch of people that don't necessarily share your interests and values. It also undermines Constitutional principles: that there be no court superior to the U.S. Supreme Court, and no law superior to the laws of Congress except the Constitution itself.
I agree there are some Constitutional issues, but I think not fatal ones. It would certainly not be possible to agree to an arrangement where e.g. a U.S. Supreme Court case could be appealed to the ICJ. But it would probably be possible to agree to an extradition agreement, by which an American court, following procedures passed by Congress, could extradite someone under its jurisdiction to the ICJ (there are already many extradition agreements with specific sovereign countries, which might be a constitutionally-significant difference, but might not). The case you link (Medellin) didn't hold that anything was unconstitutional: it simply held that the treaty the U.S. had signed in that case hadn't been implemented by Congress yet, so had no force of domestic law, and couldn't acquire such force of its own accord. They didn't hold that Congress couldn't pass implementing legislation, just that it hadn't.
You're right, I seemed to remember Medellin raising some of the Constitutional concerns, but reading it again now it was based entirely on whether the treaty in question was self-executing.
> Prosecutions for war crimes have happened both for winners and losers of wars.
Technically true. But to date the ICC has only ever prosecuted Africans. This can be seen as a continuation of Europe's colonial tendencies to treat African nations as inferior sovereigns.
Milosevic was prosecuted by the ICC and died in jail four months before an almost certain conviction. The Pinochet case predates the ICC, but although the British government intervened to temporarily "halt" his extradition to Spain, the speed of his subsequent exit (he flew back to Santiago within hours) suggests the move was a political one to push things back to Chile, where Pinochet faced subsequent prosecution and died under house arrest.
So some cynicism is certainly deserved, but it is probably more accurate to say that the more political influence someone has, and the more dirt they hold on other countries, the more protracted the entire judicial process becomes.
http://en.wikipedia.org/wiki/International_Criminal_Court
Of course the chances of countries being signatory to (and having ratified) that particular treaty are roughly proportional to the inverse of their activities regarding such crimes in the recent past.