I'll do one better and have the last chief of NATO explain it for you:

The background was that President Putin declared in the autumn of 2021, and actually sent a draft treaty that they wanted NATO to sign, to promise no more NATO enlargement. That was what he sent us. And was a pre-condition for not invade Ukraine. Of course we didn't sign that.

The opposite happened. He wanted us to sign that promise, never to enlarge NATO. He wanted us to remove our military infrastructure in all Allies that have joined NATO since 1997, meaning half of NATO, all the Central and Eastern Europe, we should remove NATO from that part of our Alliance, introducing some kind of B, or second class membership. We rejected that.

So he went to war to prevent NATO, more NATO, close to his borders.

https://www.nato.int/cps/en/natohq/opinions_218172.htm

Hope that clears things up for you.