"Attacking a NATO member will bring the US (and all other members) into the conflict. In which case, yes, American youth (soldiers) would go to war. Putin has exercised very questionable judgment in Ukraine, and has paid a grievous price. But I don't think he would take on a NATO member. That would be suicide, and I think he knows that. Russia would be crushed.

So I'm not too worried about that scenario."

If Ukraine were to somehow fall to Russia, I don't think Putin would directly attack a NATO member next. What he would do though is try and undermine their political and social structures. He would choose one of the weaker NATO border countries and heavily support opposition groups in that country. These opposition groups would become radicalized, and he would try and start a civil war. He could then send in mercenary groups to support the radicals and be able to deny Russian interference. Maybe throw in some self-sabotage (like blowing up a Russian Orthodox church in the country and then he can use that as a justification to send in Russian troops.

All of that would be gradual and be like the boiling frog. At what point does article 5 get invoked?