I am not fully aware of what transpired there.
Nutshell:
The Milosevic regime was engaging in ethnic cleansing / genocide and crimes against humanity in Kosovo.
UN Security Council voted resolution 1244. This resolution called for a peacekeeping force to move in to stop Milosevic's aggression.
NATO carried out that mission with UN mandate.
Milosevic was eventually apprehended and shipped to Den Haag where he was prosecuted before the international court of human rights and found guilty of warcrimes and crimes against humanity.
@Estro Felino likes to pretend as if this situation is comparable to Ukraine with Putin taking on the role of NATO in Kosovo to go and "save" the people in Donbas.
In reality, off course, this is nothing short of perverse. There were no UN resolutions, mandates, ethnic cleansing, genocides, etc present at all.
However, there ARE accusations of warcrimes in Den Haag at the address of Putin and the Russian army.