Just days after the deadline set by U.S. Secretary of State John Kerry for President Bashar Assad to begin a political transition in Syria, fighting is raging in Aleppo, the last urban stronghold of the rebels opposed to the regime. The United States, however, is not coming to their rescue, because the wrong kind of rebels are involved in the battle. In terms of helping to end the war, the U.S. inaction may be worse than the scenario touted by Donald Trump — an alliance with Russia to defeat Islamic State — but it’s more politically acceptable.

The group formerly known as Jabhat al-Nusra attempted to rebrand itself as Fatah al-Sham at the end of July, ostensibly distancing itself from its mother organization, al-Qaida. It now appears to be the driving force behind a rebel attempt to break the Syrian army’s siege. The desperate attack — supplies have been running out in eastern Aleppo — appears to have fizzled: With Russian air support, the regime forces and their Iranian and Lebanese allies are pushing back and holding the siege, which cuts the rebels off from Turkey.

In any case, since its failed coup, Turkey appears to be more aligned with Russia than with the rebels. President Recep Tayyip Erdogan has stepped up his anti-American rhetoric, accusing the U.S. of complicity in the coup attempt.

For its part, the U.S. hasn’t bought the al-Nusra rebranding and still considers it a terrorist group. That means it cannot defend it. In May, Kerry demanded that a political transition from Assad’s regime start by last Monday.

“Either something happens in these next few months, or they are asking for a very different track,” Kerry said. Yet the “track” appears to be exactly the same as before: Kerry has only suggested that Russia “restrain both itself and the Assad regime from conducting offensive operations, just as it is our responsibility to get the opposition to refrain from engaging in those operations” — advice that both sides are ignoring. In the absence of support, the so-called moderate opposition is coming to rely increasingly on the jihadis for help.

That’s a nasty situation for the U.S., as Michael O’Hanlon of the Brookings Foundation pointed out:

“The United States doesn’t have major allies on the ground, except the Kurds, with real military potential. And yet, we are still hoping to simultaneously defeat (Islamic State), defeat the Nusra Front, and replace Assad. That approach just doesn’t make sense.”

The U.S., however, doesn’t have a reverse gear: Kerry can’t tell Russian President Vladimir Putin that the Obama administration no longer wants to get rid of Assad. Putin is in a similar predicament in eastern Ukraine: Although propping up the secessionist “people’s republics” there is a costly, futile exercise, he cannot abandon them without appearing to cave in to Western demands.

In Syria, Putin is holding a much stronger hand. Unlike the U.S., he’s backing a capable force that — with Russian and Iranian support — could take on the insurgents. The opposition has probably missed its best chance to negotiate a political transition now: Since the U.S. is standing back, and so is Turkey, Assad, Putin, Hezbollah and Iranian forces are going to pummel them.

Putin has maintained from the start that the Syrian conflict is binary — a war between the legitimate government and an assortment of terrorist groups. Now he’s about to bend reality to this inaccurate description: If the Assad forces take Aleppo, the war will be essentially binary — between the Assad coalition and Islamic State.

For Putin, this is preferable to a negotiated solution: The defeat of the opposition groups in Aleppo would make the U.S. reluctance to leave Assad in power irrelevant. There would be no alternative.

At the same time, there’s not much incentive for the U.S. to embark on a “different track.” No one would gain from increased American support for the hard-pressed opposition — neither the U.S. itself, nor even its European allies, in constant danger of new waves of refugees: An intensification of fighting would force more people to flee. In addition, U.S. backing for the anti-Assad Kurdish militias is further straining America’s relations with Turkey.

The U.S. appears to be allowing Assad, Putin and Iranian generals to implement their plan. If they succeed in taking Aleppo, they will face Islamic State without U.S. support. They probably would be able to defeat the terror group if they’re not forced to fight on several fronts.

It would take far less time if the U.S. gave up on alternatives to Assad and joined the coalition. Since that’s politically impossible barring a Trump victory in the presidential election, the war will go on and claim more lives — but it’s difficult to see Islamic State withstanding a concentrated Russian-Iranian assault in the long term.

If the U.S. allows this scenario to unfold, its credibility as an international arbiter will inevitably suffer. Eventually, it may have to watch from the sidelines as Assad and Erdogan gang up on the Kurdish militias that have received open American backing. It may be forced to look on passively as a victorious Assad unleashes reprisals. It will definitely face a gloating, empowered Putin.

Yet inaction may well be the less costly option — at least no public reversals would be necessary, and someone else would shoulder the burden of fighting.

The U.S. can only hope that regime forces will somehow be defeated at Aleppo. That, judging by events on the ground, is a thin thread to hang on to.

Leonid Bershidsky is a Bloomberg View columnist. He was the founding editor of the Russian business daily Vedomosti.

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