If Ukraine is in the process of obtaining NATO membership it may go a long way to getting the Russians to step back from a full invasion. If they do invade and annex all of Ukraine that puts Russian forces on a very tense Polish border. The Poles are not going to be the least bit receptive to having Ivan at their border again.
Because it worked out so well the last two times.
Poland could very well be a NATO member. The Russians will throw a hissy-fit, talk smack in the international press, decry Western hegemony in Eastern Europe to the UN, but they won’t go much beyond diplomatic and informational means to stop it, they may even go economic with the pipeline, but I doubt it. Ukraine is a different story. Ukraine will not become a member of NATO. Europe relies too heavily on the Russian and Azerbaijani pipelines for their oil and natural gas. Russia can cut off their supply of the former and destabilize the latter enough to where the flow is interrupted (they already have Armenia in their pocket and Georgia is obviously quite vulnerable to them as well). This would sink Europe. Their governments won’t allow it, not in a million years.