It was announced in the news that with the delivery of Patriot missile systems in Ukraine, it would take USA military to man those weapon batteries. That is boots on the ground. Now imagine Russia striking them and killing 15, 50, 100 Americans. Then what? This escalates into Vietnam over night, only with nukes lurking over the horizon.
I do not put Putin using nukes as being a long shot. I can see him easily using them. His whole behavior with this Hitler style attack upon Ukraine has been irrational. Anything can happen, and most likely will.