I think that bombing Serbia was probably the rupture that has led us to the terrible situation we have in Ukraine now.
The fact that you call it "the issue of Kosovo" is telling.
The problem here is that when you give a defensive alliance a mandate to defend entities that are not covered by it's mandate it becomes a de-facto offensive alliance.
There's a question of semantics here of course. We're contrasting aggressive and defensive military action. I don't see how characterising military action in defence of others as aggressive is a useful or meaningful characterisation. It's defensive, just not defensive of yourself.
Also let's look at the objectives. The intention in Kosovo wasn't to invade Serbia but to end ethnic cleansing. The fact it took force to achieve that was a means to and end, not an end in itself. Without the ethnic cleansing there would have been no military action, so it wasn't about territory or aggression.
If the way you choose to prevent genocide is by marching an army into a foreign country, that's called "offense". Your argument is sophistical.
Actually, it does. Kosovo and Iraq are clear evidence that NATO is not a defensive pact. Neither attack was preceded by a judicial ruling that genocide was occurring (and genocide is very difficult to prove).
I'm afraid those attacks seriously undermined NATO's moral authority as a "defensive pact".
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