Airborne and light infantry, yes. Heavy stuff, though, usually goes via ship, and Serbia is land locked and we control the seas. Plus, there is the question of air cover. Russia could get a ferw planes into Serbia, but not like NATO could via the U.S. Navy and NATO airbases in Italy. So, their main military option would be light / airborne infantry with little air cover. That’s fine if the West blinks and doesn’t fight. These forces are okay against the KLA and such. But, if the west up’s the ante and stands it’s ground, Russia would be at a severe military disadvantage.
Oh, but we wouldn’t do anything to stop them.