Ukrainian troops have made huge headway routing the separatists in the east. They are in the process of choking off the cities of Luhansk and Donetsk, to which many of the separatists have retreated. The Ukrainian military appears primed to besiege the cities. As Ukraine has gained, Putin has prepared Russia for invasion: as of Monday, Ukraine says there are 45,000 combat-ready troops are amassed at the border. The chance that Russia invades is certainly going up.

But it’s still Russian President Vladimir Putin’s Plan B. Here’s why that’s the case … and what could change his mind.

At all costs, Vladimir Putin wants to keep Ukraine in Russia’s orbit. That requires two guarantees: 1) that Russian influence over southeast Ukraine will remain intact, and 2) that Russia has a de facto veto over Ukrainian NATO membership. The way he gets these guarantees is through deep federalization, where Ukraine’s eastern regions can set their own foreign economic policy and veto approaches to NATO. Of course, Ukrainian President Petro Poroshenko is completely unwilling and unable to concede this. So Putin can get it the hard way (intervention), or the harder way (invasion).

Sustained intervention is Putin’s current and preferred approach, where he can foment enough instability through the separatists that a unified Ukraine cannot pull away from Russia. The “long game” is more Russian arms provisions and economic pressure until Kiev is forced to accept a deeply Russia-influenced federal system.

Direct invasion would come with a much more staggering price tag. First and foremost, the Russian people are opposed. While the vast majority of Russians agree with Putin’s current Ukraine policy, a recent Levada poll revealed that 51 percent of Russians oppose an invasion and only 29 percent are in favor. Taking and holding the territory would come at tremendous economic cost, and once Slav-on-Slav bloodshed gets broadcast back in Russia, Putin’s popularity would plummet. Also, overt invasion would elicit a move towards much more acute, Iran-like sanctions from the United States, with Europe more willing to get into lockstep with them.

But even though Putin still prefers the intervention route, it makes sense for him to brace for invasion. The mere threat has strategic benefits: it’s primarily a deterrent against Ukrainian President Petro Poroshenko, demonstrating the consequences for a siege of Donetsk and Luhansk. In addition, it makes the West focus on the question of ‘Will Russia invade or not invade?,’ drawing attention away from the blurrier forms of intervention that Putin is already engaging in.

Make no mistake: Putin’s preparations are not just a bluff to deter and obfuscate—it’s also his backup plan. If Ukraine backs down on a siege of the rebel stronghold cities — or fails — we are still in the long game of intervention. That seems most likely, given the difficulties of the Ukrainian military engaging in street-to-street urban warfare. But if the Ukrainian military does rout the separatists, invasion may be the only card Putin has left.

How would he go about invading? He has already laid the groundwork: it will be under the guise of a peacekeeping mission, responding to the rebels’ calls for humanitarian assistance. There are reports of Russian vehicles retrofitted with peacekeeping insignias already present on the border. Putin did “respect” the Donetsk and Luhansk referenda declaring their independence from Ukraine: he could conceivably invade and still make the incredible claim that it is not truly Ukrainian territory. The peacekeeping pretext would not soften the harsh American response. But perhaps it could keep some European countries dragging their feet—and other BRICS countries firmly on the sidelines.

For now, Putin’s best option is still intervention over invasion. But Ukrainian military gains could shift his calculus. Invasion is more than just a bluff and a bargaining chip — the risk is very real.

PHOTO: A Ukrainian serviceman uses a pair of binoculars as he guards a checkpoint near the eastern Ukrainian town of Debaltseve, Donetsk Region August 6, 2014. REUTERS/Valentyn Ogirenko