We would not wage nuclear war over Kosovo ... so there is really no annihilation risk here.
That said, Russia’s military options are limited. Putin could put in airborne troops pretty easily, but that’s about it ... and they would have no aircover as they have little basing options for their warplanes.
Now, Russian airborne troops on the Serb / Kosovo border would still freak out alot of Western politicians ... and may be enough to cause them to blink and back down, so who knows.
Russia does have some leverage, though, in it’s energy resources and what it supplies to Europe. We’ll see if that card gets brought up in the near term.
Couldn’t they just fly troops into Serbia and when they get enough, invade? If Serbia wants Russia’s help wouldn’t they allow them to build up there? I mean, we didn’t just march into Iraq from San Diego. We flew them to Kuwait.