As the Ukrainian army chases separatists from the strongholds they've held for months, Moscow has barely said anything—despite its springtime rants about protecting Russians wherever they may be in the world. Instead, as Kiev's armies are finally uprooting the rebellion in places like Slovyansk and Kramatorsk, Russian Foreign Minister Sergei Lavrov made a tepid and noncommittal statement, saying that "a quick end to the bloodshed is in our common interest."

As I wrote back in May, now that he's sown chaos in Ukraine—but uneager to participate in someone else's civil war—President Vladimir Putin has thrown the rebels under the bus. In June, rebel leader Igor Strelkov said that "Putin betrayed us," and that betrayal has only deepened as Kiev launched its all-out offensive last week. Moscow, having started all this, has offered no help to the rebels.

The betrayal, it seems, may be even nastier than that. According to a Ukrainian security council spokesman, the Russians have sealed their border, shutting down three key crossings. Not only are they not letting men and materiel into Ukraine from Russia, but they're also blocking men and materiel from flowing in the opposite direction. That is, the very men that Moscow has riled up to the extent that they have taken up arms and are ready to die in order to get the region out of Ukraine and into Russia are not welcome to seek refuge in Russia. (Not even, it seems, the ones originally from Russia.) A group of 300 fleeing rebels reportedly even came under fire by the Russians as they tried to escape into Russia.

The Russians haven't confirmed or denied these Ukrainian reports, but it would not be out of step with Russian military history: The Red Army was notorious for its use of so-called barrier troops that were stationed behind active combat troops to prevent retreat. They became especially notorious in World War II when, drowning in the meatgrinder of the German advance, ill-equipped and poorly trained Soviet soldiers (many of them volunteers) were shot for retreating.

And given the outsized shadow cast onto the Ukrainian conflict by the rhetoric of World War II–era Russia, using its tactics wouldn't be all that surprising.