[Airborne Rifles]

I actually think NATO would respond strongly, though it would take time. Right now the Russians can overrun the Baltic States, though that is more difficult now with the heavy multi-national battalions rotating through those three countries, and a US armored BCT in Poland. I would think a war between Russia and NATO would unfold something like this:

-Surprise Russian push simultaneously into the three Baltic States in conjunction with "hybrid warfare" supplied by Russian minorities in those countries. The Russians overwhelm the Baltic militaries as well as the rotating NATO forces, though the fight is bloody.

-Next the Russians pause and dig in. They really don't have the capacity to go any further.

-Meanwhile, NATO gathers strength (mostly American, but also German, British, French, etc) in Poland for a counterattack while the air forces gain air superiority, with difficulty. This is the most dangerous point, because the Russians, knowing they can't win if they give NATO time to build up a powerful counteroffensive, will likely execute their strategy of "escalate to de-escalate," meaning they would launch a limited nuclear strike at some military target like a naval task force or assembly area in eastern Poland. Their thinking is that they can push across the nuclear thresh hold to bring NATO to the negotiating table, since NATO will be unwilling to cross the same threshold.

-Regardless, if the Russians think this course is too risk and decide not to go nuclear, or if a limited nuclear strike fails to bring NATO to the table, then after several months NATO would launch a powerful offensive into Kaliningrad and the Baltic states to reclaim the lost territory.

Regardless, I think this whole potential conflict is great fodder for SPMBT scenarios, especially the eventual NATO counter-offensive!