The people of Poland

[headquarters]

Imho - and h being for humble :

The Poles would get into a civil war. There was massive popular support for the pro democratic and by assosciation western aligned factions from the late 1970s. A huge chunk of the Poles were actually quite anti Russian in the sense that they wanted them out and their Polish puppets deposed.Solidarinocz was a popular movement that in true revolutionary fashion managed to outwit,outlast and and then oust the secret police of an oppressive regime. Imhumbleo this would be the dominant factor if a NATO invasion supported it and did not alienate it through say bombarding or nuking Polish cities etc.

On the other hand, a fair share of Poles were card holding party members or inclined to lean that way. They would be in a hard place if the system they had invested their lives and themselves in was to be dissolved. The USSSR gave aid and had a finger in every pie about Polish reconstruction post WWII.
I believe it safe to say that this would be serious contender.

A Nato invasion of Poland in the T2K timeline would mean that these factions probably would come to blows and become belingerents of the Bloc or NATO forces.

So a typical grey area answer from me - yes AND no. The Poles would fight NATO. But NATO would be seen as liberators by many too.