That’s an act of war, so Russia would find itself at war with NATO.

NATO would have every reason to show restraint; they would simply crush the Russian troops in Ukraine. Including Crimea.

It is worth noting that no country has ever attacked NATO in the North Atlantic area covered by the treaty. Individual NATO countries, yes, outside the area where NATO defense policy applies, like the Falklands. But not in Europe.

That’s sort of the whole point.