Military leaders were charged with war crimes because they were torturing the enemies during and after the war. Which was illegal.
Slobodan Milosevic was the president of Serbia from 1989 - 1997. He was the main defendant for one the biggest war crimes. Milosevic was trialed and convicted of war crimes and genocide.
Yugoslavia broke apart due to a series of political upheavals and conflicts. After 1980 republics of the social federal republic of Yugoslavia slit apart but un-solved caused issues. It started on May 4, 1980 and ended April 27, 1992.
One condition that led to the conflicts was that Slobodan Milosevic refused to listen to demands from the other countries. Secondly the central government wasn't strong enough to keep them all together.