If Russia went into Eastern Ukraine and stayed there, then I could see NATO forces deploying into western Ukraine at a later date, not in order to engage Russian forces, but in order to 'defend Ukraine' against Russia, in a manner not dissimilar to east and west Germany in the cold war.
Honestly though I don't see Russia deploying into eastern Ukraine in force any time soon, not unless something major changes in the meantime.
Either which way, neither NATO or Russia wants to go to war with each other and both will take as many measures as they can to avoid such a thing, however both will also do their best to outsmart the other and gain an advantage wherever they can. It's old Cold War tactics, and we didn't go to war with Russia back then.
