NATO would collapse. Greeks would refuse to fight. Serbs would be a war in the middle of all this. Cypriots would refuse to fight. Turkey would likely also refuse to die in a war they could only lose from. Bulgaria would probably have a revolution. Romania and Italy and Spain and Portugal would not long suffer heavy casualties before their unpopular governments were overthrown. France more than likely also. US couldn’t fully concentrate their army as they would have to release their grip on all other sectors which in turn would be blowing up.
The possibility of premeditated war with Russia is real; this was never a possibility during Soviet times.