U.S. Democratic presidential candidate Hillary Clinton speaks at a "Women for Hillary" campaign rally in Portsmouth, New Hampshire September 5, 2015. REUTERS/Brian Snyder (Brian Snyder/Reuters)

HILLARY CLINTON has touched off an important and overdue debate among Democrats about U.S. policy in Syria. Ms. Clinton told a television interviewer last week that she favored the creation of a no-fly zone and humanitarian corridors “to stop the carnage on the ground and from the air,” some of which is now being inflicted by Russian bombers. That drew a disdainful response from President Obama, who described such proposals as “mumbo jumbo” and hinted that Ms. Clinton’s declaration was mere campaign rhetoric. Mr. Obama’s view, in turn, was endorsed by Sen. Bernie Sanders (I-Vt.), who issued a statement saying he opposed “a unilateral American no-fly zone in Syria.”

We’d like to see a substantive discussion of this question, so we’ll start by trying to clear away the chaff tossed up by Mssrs. Obama and Sanders. Ms. Clinton can hardly be accused of favoring intervention in Syria for political reasons, since she first put forward her proposal three years ago as secretary of state; it is, if anything, a politically risky stand for a candidate who must face Democratic primary voters in Iowa four months from now. Mr. Sanders can more plausibly be accused of electoral pandering. His false suggestion that Ms. Clinton was proposing “unilateral” U.S. action was evidently meant to evoke her support for the “unilateral” invasion of Iraq.

In fact, any initiative to establish Syrian no-fly zones, where refugees could gather safely and moderate forces organize themselves, would necessarily involve extensive U.S. collaboration with Turkey and Jordan, Syria’s neighbors, as well as those nations already participating in the U.S.-led air campaign in the region. The Obama administration several months ago nominally agreed to Turkey’s proposal for the creation of a small de facto safe zone along its border with Syria. But as so often in Syria, implementation has been lacking.

What of Mr. Obama’s claim that proposals like Ms. Clinton’s are “mumbo jumbo” and “half-baked”? It’s worth considering the recent congressional testimony of retired Gen. David Petraeus, who like Ms. Clinton first advanced his ideas from inside the administration. The creation of enclaves in Syria “protected by coalition air power,” he said, was not only feasible but also essential to the political solution Mr. Obama says he is seeking for Syria.

“If there is to be any hope of a political settlement, a certain military and security context is required — and that context will not materialize on its own,” Mr. Petraeus said. “We and our partners need to facilitate it.” What’s needed is the creation of a formidable moderate Sunni force that can act as a counter both to the Islamic State and to the Syrian regime of Bashar al-Assad. That in return requires territory that is safe from aerial attacks.

One leader who clearly understands Mr. Petraeus’s point is Vladi­mir Putin, who has dispatched planes and troops to create the “context” for his preferred political outcome — which is the entrenchment of the Assad regime. Mr. Obama spoke disdainfully of Mr. Putin, as well, saying he would neither cooperate with his military campaign nor “make Syria into a proxy war between the United States and Russia.” Yet by declining to protect moderate forces from Russian bombing, Mr. Obama is advancing the end state Mr. Putin seeks. Once again, the president is overlooking the dangers of inaction; Ms. Clinton was right to speak up.