How are they going to establish bases in the country? Take a look at a map! No surrounding country is going to permit the use of their airspace. All military flights would have to be permitted. No neighbouring nations is going to permit arms and troops to be flown into Serbia to establish a base or replenish said base.
Serbia is still under treaty in relation to its arms and equipment with surrounding nations. No surrounding country is going to permit or aid any inbalance in forces or equipment in the region. No access from the sea and no access from the air. Russia is hamstrung. They withdrew their peacekeeping forces from both Bosnia and Kosovo.
I am not sure how the doctrine of Innocent Passage applies in this case, but I know for a fact that the USN uses that doctrine to enter the territorial waters of other nations in transit from one location to another.
Be that as it may, I wouldn’t put it past Putin to organize a massive airlift and dare NATO to stop it. By the time the NATO folks finished dithering and consulting, he could have several regiments and their equipment on the ground.
I wouldn't hold my breath on that. The list of countries who have not yet recognized Kosovo and -- especially those who have definitely stated that they will never recognize Kosovo independence -- might allow limited airspace use or access from the sea for the specific purpose of assisting Serbia in her defense of Kosovo. Like it or not, NATO is the "occupier" and not the native to that region.
Ukraine is on the fence, Belarus and Romania have said "no way" to recognizing Kosovo, Montenegro has said "not yet" -- but the Montenegrin people have said "no way".
It's not as cut & dried as you might think. They don't want a war, but like it or not, Russia is a lot closer to them than to us -- and they aren't going to shoot down Russian planes over this.