Any system in which the victor determines the war crimes is a justice system that will not ever work, because by its very nature it is unjust. In point of fact, all sorts of crimes are committed in war, because war is, at best, legal murder, and at worst, outright genocide. Although wars can bring out the best in men and women in regards to their courageous acts of bravery and sacrifice, so too does war bring out the worst in people, so that under the guise of wartime conditions, and far too often under this convenient cover it is used to slaughter innocents or enact cruel punishment or both.
The one thing that the world has become in modern times is that it has gotten a lot smaller, and has thereby created a world that is truly international and global in structure to which most every country in the world interacts with many other nations all throughout the world. For instance, there probably isn't a country in the world today that doesn't conduct some sort of business, whether charitable or not, whether desired or not, with the United States of America.
In 1998, the United Nations did indeed establish the International Criminal Court (ICC), located in The Hague, to which this permanent global criminal tribunal was given the power to enforce prosecution efforts against those that have committed crimes against humanity, and war crimes, to which 121 countries have, to date, ratified this treaty. The United States is one of those countries that has not ratified the treaty and appears to have no intention of ever ratifying the treaty, demonstrating its complete contempt of an international tribunal that would ever dare to even conceive that the United States could be guilty of any war crimes.
The fact of the matter is, there is just one superpower in the world, which acts and behaves as the world's global policeman, and that country spends an inordinate amount of money, materials, and men prosecuting its own version of justice throughout the world, to which, if it so desires, it deliberately targets, destroys, and kills whomever it may so select, no matter the consequences. This country answers to no international tribunal, and so it also answers to no other country, accepts no criticism, and believes that what it does is always right, even when it clearly is in the wrong.
The United States won't be a signatory to the ICC, because it is a country that is always the victor, even in those few cases where it appears to have lost, because those don't count as an actual loss, they are instead more akin to a great eagle merely losing interest in a certain prey. America refuses to dip its flag, because it doesn't have to, and like that spoil schoolyard bully, there doesn't appear to be a soul or country around that can make it do so.
There are war crimes committed every day, some of those, no doubt are committed by America, but if America considers itself above the law, there will never be true international justice, and without an independent board, made up of members not beholden to any State or Government, and able to fairly prosecute all countries and its constituents, this will remain a world in which the winner judges the losers, and thereby validates justice in the manner that might is right, and civilization will be worst off for it.