I think we would be required to act if they entered Poland/Baltic states, what with that whole NATO thing. Of course, that's not Russia's plan. The Russians view Ukraine as their sovereign right, a sphere of influence which they have held since the breakup of the Polish-Lithuanian Commonwealth. Seeing Ukraine act independently and, god forbid, actually make their own foreign policy decisions, is seen as an affront to the grandeur that is Russia. So the whole point in this exercise has been to chastise Ukraine and grab what they can from it before it disappears from their grasp completely. The USA won't go to war over it, and we shouldn't. WW3 should not be begun because we want to limit some Russian dreams of a renewed sphere of influence in Europe. If they touch NATO, then yeah, we're treaty bound to assist, but they won't. They're just trying to regain some of the prestige they once had, to overcome the national embarrassment that was the breakup of the USSR and the stagnation/depression of the 1990s.