Surely you mean the Federal Republic of Yugoslavia? Unless you're implying that was a sham political cover for naked Serbian self interest. Perish the thought.
NATO was defending Kosovo from a war of aggression and ongoing genocide.
NATO was defending Kosovo from a war of aggression and ongoing genocide.
Except that Kosovo was not a part of NATO so you can't say that NATO is a defensive alliance unless you are going the Roman way of pre-emptive defence[1].
[1] All Roman wars were officially defensive because they believed gods would not support an offensive war