[PapaBear3625]

Remember when the Russians could not secure flyover rights for their paratroopers during the Clinton war? I doubt they will be able to achieve that now that Bulgaria and Romania are firmly in the NATO camp.

So they covertly pre-position equipment using commercial transport, and a few tens of thousands young fit Russian males decide to vacation in Serbia

[Tommyjo]

No they can’t. All flight plans would have to be filed. A bit of a giveaway as to their intentions. They would have to use heavy lift aircraft to get equipment in which would also raise alarm bells. No NATO country surrounding is going to allow use of its airspace for such actions. How would they maintain resupply of said troops?