War crimes only exist for the militarily weak nations. Look at what the Soviets did to German POWs after WW2. Not a single member of the 1st SS division Liebstandarte made it back to Germany after being deposed in Moscow. Several thousand Heer soldiers never made it back to Germany after capture by the Soviets.
How about the US actions in Vietnam and Cambodia? We literally killed millions of civilians. Entire villages would be bombed if they were suspected of harboring Vietcong. Or how about Iraq I when we deliberately targeted water plants, power plants, sewage plants, and other civilian infrastructure? Or Iran-Iraq where there is documented evidence that we funneled chemical and biological weapons to Iraq and greenlighted their use against the Iranian army? Or in Soviet Afghanistan when the Soviets would literally raze an entire village for killing a Soviet officer?
The bottom line is that war crimes only exist to punish the losers of wars. And even then, only if the loser is weak enough and the prosecutors strong enough. If Nazi Germany had won (or drew) WWII, there would have been no Nuremburg. Hell, Allied officers would have been on trial instead of German officers for violations against the German peoples. Look at what Israel can do because it has a powerful ally in the UN. Israel can basically ignore any allegations of war crimes against it. This is because the US is a powerful backer and because Israel has not lost any wars. What Israel does, on a daily basis, far outstrips what Milosevic and his cohorts did in Serbia. What's the difference? Serbia incurred the wrath of NATO and lost it's powerful backer in Russia. It's really as simple as that. If the ethnic cleansing happened during the '70s or '80s, Slobodan would be hailed as a hero in the East. Sure, the West may condemn him but he would never serve any time. He could effectively ignore any condemnation brought onto him by the Hague and UN because he has an 800-lb gorilla on his side.