Russian President Vladimir Putin had his “mission accomplished” moment Monday, announcing that Russia would withdraw its main forces from Syria after they turned the tide in President Bashar al-Assad’s struggle against Syrian rebels.

The announcement partly explains why Putin has been supporting a cease-fire and truce talks over the last month: His goal is to consolidate the gains he and Assad made together. From a purely cynical perspective, the operation has been a fairly impressive success for Putin: Bomb intensely to create a humanitarian crisis while your troops advance, then negotiate peace to look like a good guy while assuring that the other side can’t fight back without violating the truce. And accomplish all this while strengthening your bargaining position vis-à-vis the U.S. and Europe.

But the announcement also makes it explicit that Putin has no interest in giving Assad the support he’d need to take on the forces of the Islamic State. By declaring victory before Assad has really confronted the Sunni militant group, Putin is saying that Russia is perfectly willing for Islamic State to remain in existence indefinitely.

Putin’s lack of interest is unfortunate insofar as Islamic State’s persistence as a functioning statelet is destabilizing for the region, bad for the world and disastrous for the victims of its brutal governance. But to be fair, no one else in the region seems very interested in taking on Islamic State, either. Putin has simply calculated that he has less to gain by trying to take it on and risking failure than by declaring victory and departing the field.

No one should have any illusions that Assad was consulted about Putin’s decision, which reflects Russian interests, not his regime’s. Assad will miss Russian troops, although he will probably still have some Russian air support, because the Russians are keeping their airbase at Hmeimim, near Latakia.

Nevertheless, Assad may well agree with Putin that attacking Islamic State would be a bridge too far. It would be extremely dangerous for Syria to seek to recover territory held by the group, even with Russian support. Assad needs all the troops he has to try to consolidate the gains he’s made against Syrian rebels, particularly near Aleppo. He needs to establish some sort of functioning government in the part of the country he holds.

That means the Russian announcement is a significant win for Islamic State. Putin is guaranteeing that Assad will have no choice but to seek a cold peace with the group, respecting its border in the hopes that it won’t try to invade the territory Assad has recovered or make a run at Damascus.

From Islamic State’s angle, a temporary, cold peace with Assad looks appealing. The group can’t govern millions of Alawites loyal to Assad. But because it considers them infidels, it would have to try to convert, expel or kill them if it somehow managed to defeat the regime.

More to the point, Islamic State can’t fight on all its fronts simultaneously. It faces a protracted struggle with the Iraqi government, which is slowly (and not all that surely) trying to reconquer majority-Sunni areas of Iraq. Islamic State is likely to face long-term conflict with Turkey, which doesn’t want a terrorist state on or near its border, and with Kurds of whatever stripe, who will fight over every inch of territory.

Under these circumstances, short-term conciliation with Assad looks pretty good for Islamic State.

Meanwhile, Putin and Assad have done the group the tremendous favor of knocking back all the other Syrian rebels. If you’re a Syrian Sunni who hates Assad, you’re running out of options for joining a credible opposition. It may not happen right away, but Islamic State could easily come to look like the only serious Sunni alternative to Assad. That could mean manpower and even support from frustrated Sunni Syrians.

Eventually, becoming the only Sunni force opposing Assad could also mean indirect support from Saudi Arabia and the Gulf principalities. Those states see Syria as an Iranian tool, an illegitimate outpost of Shiite geopolitical influence. Structurally, those states’ interest is with Sunnis who oppose Assad and Iran, no matter how repulsive they might be.

To be sure, Putin isn’t trying to help Islamic State. He just doesn’t care enough about it to take the risk of entering an Afghanistan-style quagmire in Syria. Getting out fast will burnish his credentials as someone who understands how to make war in the Middle East. He leaves Syria having achieved a limited objective cheaply and quickly. Russia looks stronger than before. Assad will continue to need Russia as much as ever, and won’t become the kind of client that can make infinite claims on its patron, as happened with U.S.-backed governments in Afghanistan and Iraq.

It’s been a nasty business. But from Putin’s perspective, it’s been well worth it. It’s one more reason to take Putin seriously as a danger to the European state system — as if we needed one Crimea.

Noah Feldman, a Bloomberg View columnist, is a professor of constitutional and international law at Harvard.

(c) 2016, Bloomberg View · Noah Feldman