Belorussia attacking a NATO country ends at best in it being occupied by NATO, at worst in the same war between NATO and Russia that Putin don't want in the first place.
Armenia is a member too, but Putin didn't help them in Nagorny Karabakh, saying it was Azerbaijan territory by international law. I beleive it was a demonstrative punishment for Armenian 2018 "velvet" revolution.
Kazakhstan is a member too. Putin used his army in Kazakhstan in January after Tokaev said that protesters are supposedly foreign terrorists.
Putin will not directly help Lukashenko against NATO countries on their territory, but is well capable of throwing his army to "protect" Belorussia from retaliation, including anti-air warfare.
This. A way worse result for Putin than Ukraine slowly becoming NATO-aligned is a sudden NATO-enforced regime change in his other (and currently reliable) European buffer state.
