This is what bothers me about predictions of a Russian invasion: NATO isn't a pushover.

Russia may be able to steamroll through Ukraine and then dig in. This isn't impossible — Russia has world-class surface to air missile systems, world-class special forces, hypersonic missiles, and a massive left-over Soviet arsenal (and before someone points out that cold war technology is obsolete, keep in mind that F16's and M1 Abrams tanks — workhorses of the American military and currently in heavy use — are also cold war weapons).

But even if it could launch a successful invasion (presumably install a friendly government in Kiev and dig in permanently), the battlefield won't be confined to Ukraine. NATO navies could still attempt a blockade of Russian ports. Russian flights would be interdicted over European airspace. Russian trade would take an enormous hit from new economic sanctions.

It's more logical to assume that whatever maneuver they plan would proceed slowly and cautiously. For example, they could move forces into Donbass in order to fully secure its independence. A declaration of war over such a small region would be more difficult for NATO to justify. But even in that scenario, NATO could escalate its presence in Ukraine in response — the exact opposite of Putin's goal.