The civilians fleeing Aleppo don’t prove definitively that, with Russian backing, President Bashar al-Assad will win the Syrian civil war. But it’s certainly time to game out that scenario and ask: What would victory look like to Assad? And what will happen to the other regional actors engaged in this fight?

The decisive element to consider is whether Assad needs to defeat Islamic State to be a winner. If the answer is yes — and if Assad could do it — the world would probably breathe a sigh of relief, and accept Assad’s victory, despite its extraordinary human costs and egregious violations of human rights.

But Assad will probably calculate that he doesn’t need to beat Islamic State, just contain it so that it doesn’t constitute an existential threat to his regime. That would put Islamic State well on its way to becoming a statelet, accepted by its neighbors for lack of will to defeat it. The long-term consequences for the world would be high, but Assad’s regime would be substantially better off.

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For now, Assad appears to be moving toward at least a limited victory over the ill-organized Free Syrian Army forces around Aleppo. It isn’t rocket science. He’s combining intense air support from Russian planes with ground forces drawn from what remains of the Syrian army.

The Battle of Aleppo has been going on since 2012. What’s changed in this round is the intensity of Russian airstrikes. Republican presidential candidate Ted Cruz may have said that he wants to carpet bomb Islamic State to find out if sand can glow in the dark, but it’s Russian President Vladimir Putin who’s following a version of that strategy against the Syrian opposition.

It’s still conceivable that the Free Syrian Army could rally, but it doesn’t look likely. That would mean Assad could consolidate control over Aleppo and whatever number of people remains there. The population was some 2.5 million before the war, and it’s certainly much smaller now.

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Over time, opposition fighters could in theory infiltrate back and attempt an insurgency. If Assad can’t spare sufficient troops to hold Aleppo, the Free Syrian Army might stage a comeback. But it would be doing so from a much reduced position, and the war-weary public might very well be unwilling to support it.

Once the formula of intense Russian bombing plus Syrian ground troops has been shown to be a success, Assad and Putin will repeat it over whatever Syrian territory remains in Free Syrian Army or Syrian Kurdish hands. It is entirely reasonable to think it would succeed again.

That will lead to a major strategic crossroads. Assad and Putin will at least be tempted to try their winning formula against Islamic State.

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Putin would love to show the world that he can succeed where the West has failed. Beating the Sunni militant group would significantly improve Russia’s global military prestige. Added to his taking of Crimea, it would make Putin the first Russian leader in more than a generation to win wars, which will also burnish his domestic reputation. It might be possible to achieve all this without Russian ground troops. And if airstrikes aren’t enough, Putin can simply blame the Syrian ground forces for being inadequate.

The upside for Assad would be a return to something not unlike the status quo before the Sunni uprising against him — but with a smaller national population with fewer Sunni Arabs, because many will remain in Turkey and Europe as refugees.

At one time, it seemed unimaginable that the Assad regime could return to national control. But that doesn’t seem quite as unrealistic now. Iran would favor and support Assad, as it always has. Now that Iran’s regional position has improved as a result of the nuclear deal with the U.S. and the lifting of economic sanctions, Iran would be better placed than ever to support Assad.

The Israelis have looked Islamic State in the face and concluded they’d rather have Assad than total chaos. Turkey had warm relations with Assad and was establishing open borders with Syria until the uprising broke those ties. As a geostrategic matter, Turkey would eventually take Assad back into the fold, whatever its continuing anger about the massacres he’s perpetrated. Even Saudi Arabia, which sees the Assad regime as the cat’s-paw of its rival Iran, would be prepared to live with Assad if it meant a return to regional stability.

The great risk for Assad in taking on Islamic State is the possibility of overreach. Even if he has enough troops to beat the militants, he might not be able to hold down the rest of the country. And if Islamic State forces flee Syria into Iraq, which would be the rational thing for them to do, they could come back and harass whatever forces Assad left behind. Unlike the Free Syrian Army, Islamic State would have a base from which to pursue an insurgency. It also has ideologically motivated troops with some combat experience. What’s more, Assad simply may not want to govern Sunnis area in Syria that have either sided with Islamic State or accepted its rule as a practical matter.

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Assad therefore might decide that he’d be better off with a de facto border that Islamic State respects out of self- interest. If he leaves the group alone and is left alone in exchange, he can re-establish some semblance of sovereignty in much of Syria, surely his No. 1 priority. Essentially, Islamic State becomes everyone else’s problem, not Assad’s.

This scenario seems to me more likely than a serious Syrian countermilitant push. It would leave Islamic State as a threat to Iraq, to regional security and the rest of the world. The possibility of Islamic State as a long-term, de facto state looms.

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