For the latter reason, Russia, Syrian President Bashar al-Assad’s main benefactor, would veto any U.N. Security Council effort to set up such a zone — even on humanitarian grounds. Russian intervention has also seriously complicated any possible U.S.-led initiative to set up a no-fly zone outside the Security Council’s purview because, as O’Malley pointed out, it would run the risk of direct confrontation with Russian aircraft — and according to recent reports, a potential ground invasion — in northern Syria.

As evidence, analysts point to a recent incident in which a Russian fighter jet bombing rebels in Syria allegedly crossed into Turkish airspace, in brazen defiance of Turkey’s threats that it would shoot down any Russian or Syrian aircraft that veer over the border. Ankara was outraged but wasn't willing to risk direct conflict with superpower Russia.

“Russia deliberately and symbolically overrode this measure because they knew Turkey could not afford to shoot down the aircraft,” said Ege Seckin, a Turkey analyst with the IHS consultancy, speaking from Istanbul. The message, he said, was that “that area is now dominated by the Russian air force.”

Whereas Russia has demonstrated it will throw its weight behind the Assad regime, Barack Obama’s administration has not been willing to take on the enormous deployment of military assets that would be required to set up a no-fly zone on behalf of its rebel proxies. “It would require suppression of enemy air defense, which would in turn require the U.S. and its allies to reach deep into regime territory and completely destroy Assad’s missile capabilities,” Seckin said. “That was difficult enough already, given the extensive capabilities of the regime, but now that Russians are in the picture, it’s looking almost impossible.”

Then there is the question of which faction in Syria would front the tens of thousands of troops needed to clear such a zone on the ground in order to counter further expansion in the area by the Islamic State in Iraq and the Levant (ISIL). Northern Syria is a fluid and increasingly fractured battleground split among a number of groups: ISIL, hard-line rebels such as Ahrar Al-Sham and Al-Qaeda-linked Jabhat Al-Nusra (the Nusra Front), the dwindling “moderate” rebels of the Free Syrian Army, non-aligned Kurdish YPG militias and the Assad regime. In a microcosm of Syria’s messy proxy war, each of these factions has a different list of priorities and a different set of foreign sponsors, so aligning all parties involved behind one of these groups is a tenuous prospect, at best.

Turkey, which is far more committed to toppling Assad than the U.S. is, would be amenable to allowing powerful hard-line groups like Jabhat Al-Nusra and Ahrar Al-Sham to build on their recent progress into regime territory. But Washington has hinted at ramping up support instead for the Kurds, who are the most effective, pro-Western rebel faction left in Syria. The U.S. is walking a delicate line with its NATO ally Turkey, however, since Ankara considers Kurdish separatism in any form a far greater security threat than even ISIL and fears anything that could strengthen the decades-long PKK insurgency in Turkey. In fact, analysts say Turkey’s interest in a no-fly zone stems more from its desire to clear out Kurdish fighters from the area than the need to safeguard its borders from ISIL incursion.

In light of those strategic dilemmas, some posit that Clinton is trying to distance herself from Obama’s much-maligned — some say overly cautious — policy in Syria that, as his secretary of state from 2009 to 2013, she was instrumental in implementing. Without overtly criticizing his possible successor, Obama hinted that the no-fly-zone talk was rooted mainly in politics. “Hillary Clinton is not half-baked in terms of her approach to these problems,” he told reporters last month. “But I also think that there’s a difference between running for president and being president.”