There's a popular new idea in Washington's foreign policy circles. America's at war with the Islamic State in Iraq and Syria (ISIS), and so is Syrian dictator Bashar al-Assad — so shouldn't Assad and the US team up? Leslie Gelb, the former president of the Council on Foreign Relations, has endorsed it. So have a number of prominent foreign policy analysts and former American officials.

Even the Assad regime thinks it's a good idea. Syrian Foreign Minister Walid al-Moualem said on Monday that his government "ready to cooperate and coordinate on the regional and international level in the war on terror."

As rules of thumb go, "Don't ally with murderous dictators" is a pretty good one (exception: allying with Stalin in World War II). And it holds in this case. Even a temporary US partnership with Assad against ISIS likely wouldn't destroy the group. In fact, it might very well backfire and strengthen ISIS. It would, however, definitively make Americans complicit in the mass murder of Syrians — an outcome that, both morally and strategically, does far more harm than good.

Can Assad even destroy ISIS?

Here's a big problem with the help-Assad case: it's not clear that he's actually strong enough to take on ISIS.

Around August 19th, ISIS launched an offensive targeted at the government-held Tabqa airbase, in north-central Syria. Assad fought hard to keep the base, directing punishing airstrikes and ground forces at ISIS. On August 24th, ISIS took Tabqa anyway.

The fact that Assad is already using airpower against ISIS, and failing, makes it hard to imagine that supplementing the Assad campaign with some US air strikes would be enough to push back ISIS. The US Air Force is obviously orders of magnitude more capable than its Syrian counterpart, but airpower can't take and hold territory on its own. It needs to be done by competent ground troops. Assad simply doesn't have the ground forces to spare, given that he's also fighting other Islamists and moderate rebels around the country.

Tabqa "shows that the regime's strategy of trying to hang on everywhere in Syria is failing," Washington Institute for Near East Policy Syria expert Jeff White told the Washington Post. "They just don't have the resources to do that."

ISIS isn't Assad's real target, anyway

If Assad can't hold on to all of Syria yet, there's real reason to doubt he'd ever prioritize retaking ISIS-held territory over fighting more moderate rebels.

For most of the conflict, the Syrian government was content to cede a large amount of Syrian land to jihadist rebels, especially ISIS. The Syrian government refrained from bombing Raqqa, the province ISIS holds, in the same way that it attacked land held by the more moderate rebels in western Syria.

if the US partners with Assad, the Syrian dictator will be in the driver's seat

Assad's detente with ISIS was darkly genius. If ISIS grew strong, Assad seemed to reason, then the United States and its allies would become increasingly hesitant to arm other rebels or to intervene against Assad. The Syrian government calculated that Washington ultimately cared more about preventing ISIS from winning than it cared about toppling Assad. He was right.

The Syrian government, then, cares far less about destroying ISIS than the United States does. As such, any "partnership" would be inherently unequal. Assad might not actually commit enough ground troops for any American-backed offensive to work. He could provide the US with bad intelligence and steer American air strikes toward other Syrian rebel groups. Or Assad could extort the United States, threatening to back off against ISIS unless the US promised to weaken the Western pressure on his regime.

In short: if the US partners with Assad, the Syrian dictator will be in the driver's seat. And Assad has no interest in actually winning against ISIS, because that would leave the US to turn against him, but every interest in seeing the US conflict with ISIS stretch on in perpetuity.

How helping Assad could backfire — in both Iraq and Syria

Helping Assad could also convince Sunnis in both Iraq and Syria that ISIS is the only group that's interested in protecting them from Shia oppressors — and that the US is the enemy of Sunnis.

White House deputy national security advisor Ben Rhodes has warned of this. "Joining forces with Assad would essentially permanently alienate the Sunni population in both Syria and Iraq, who are necessary to dislodging [ISIS]," he told the New York Times.

Sunnis in both Iraq and Syria hate Assad, and for good reason. Assad is Shia (more precisely, Alawite, a Shia subgroup) and privileges Syria's Shia minority at the expense of its Sunni majority. His government has slaughtered thousands of Sunnis during the civil war as part of a deliberate strategy to turn the war into a sectarian conflict.

Sunni support is the foundation of ISIS's power

Right now, the US can credibly say that its anti-ISIS campaign is in support of the Iraqi government — which in theory, if not in practice, is supposed to represent all Iraqis. And the US is publicly pushing for the Iraqi government to be more inclusive of Sunnis.

But if the US starts cooperating with Assad and the mostly-Shia Iraqi government, it'll look to a lot of Sunnis like the United States is taking the Shia side in a regional civil war. Assad's close relationship with Iran makes this much worse: Sunni Iraqis hate Iran, which they (correctly) believe is trying to turn Iraq into a Shia-sectarian client state.

Sunni support is the foundation of ISIS's power. They've taken pains to reach out to Sunni civilians in Iraq and Syria, setting up medical clinics and policing neighborhoods to shore that support. So long as ISIS has at least tacit support from the local population, it'll be impossible to dislodge it from Iraq, and possibly Syria as well.

Assad's line, according to University of Vermont's F. Gregory Gause, is that it is the "guardian of [Sunni] interests against the sectarian governments in Damascus and Baghdad." Gause calls this positioning "one of its great strengths at the propaganda level." By partnering with Assad, the US would make this great strength even greater.