MAD would prevent escalation but there would still be a fight. The United States has troops in Poland, if Russia invades, I would envision some kind of tit for tat escalation but would guess it would stop way short of nukes. But I would highly doubt that Russia would pick a fight with nato directly. In this case with Ukraine, it was clear to all parties involved that Russia could invade without other countries committing troops to the fight. I don't think Putin believes he could successfully win a fight with the west and I doubt he would want his legacy to be the man that caused the downfall of Russia.