Russia has always been a pain to invade but we can only imagine how things will turn out with newer technologies. I don't think NATO would want another war at this point in time, considering how they've been dragged into America's War on Terror. This whole talks of Trump complaining about America's contribution to the military alliance also isn't helping them.

NATO might attempt several missions in the areas annexed by Russia though I don't see them occupying the place themselves. It'd probably more like supporting Ukrainian forces in recapturing the area.