As people have already said. NATO is a defensive alliance, not an aggressor. If Russia built up troops on NATO borders, you can bet your house on them responding in kind, but Russia didn’t, so no build up was necessary.
The build up of troops can be defensive. My point being, when nations call his bluff or show force, he bottles it. When we let him probe weakness, he attacks.
A show of solidarity and force would have likely forced Putin to back down. It should have been done as far back as 2014. But that’s my opinion. I’m hoping we don’t get the chance to test the theory ever again.