THE MUDDLE that is President Obama’s policy on Syria has grown still muddier. On Tuesday the president backed away from a “red line” he had drawn on the use of chemical weapons by the regime of Bashar al-Assad, setting the threshold for proof of a violation in such a way as to virtually exclude the possibility that one could ever be confirmed. Yet that same day his aides leaked to The Post and other news organizations the news that the president might soon reverse his long-standing opposition to providing Syrian rebels with arms. And the administration readied yet another effort to persuade Russia to abandon its support of the Assad regime in favor of a negotiated political transition.

Can any coherence be found in this? A charitable interpretation might be that Mr. Obama wishes to avoid immediate U.S. intervention but wants to pressure Moscow into changing its position by letting it be known that the alternative is greater U.S. support for the rebels. If so, Mr. Obama is being too clever. His weak and legalistic words about the need to verify a “chain of custody” on any chemical-weapons use and his declaration that even a hard confirmation would lead only to a “rethink [of] the range of options” simply invite further chemical attacks.

As Free Syrian Army commander Salim Idriss — the man the administration is counting on to unify the moderate opposition — put it in a letter to the president: “Assad is not taking your carefully phrased condemnations as warnings, but as loopholes, which justify his continued use of chemical weapons on a small, strategic scale.”

As for Russia, ruler Vladi­mir Putin has offered no public hint that he has any inclination to reverse his support for Mr. Assad. It’s not just that the Kremlin has interests to protect in Syria; Mr. Putin’s priority is to prevent what he views as another U.S.-sponsored regime change. Even were he to decide to cooperate with Mr. Obama, it’s doubtful that Mr. Putin could induce the Assad clique and its principal backer, Iran, to give up what the dictator himself has called a fight to the death.

A slim chance for a political settlement may still exist but only if the United States and its allies take measures that decisively, and relatively quickly, shift the momentum of the war. Only when the Assad army is defeated and the regime crumbles will a deal be possible. Supplying arms to the rebels, as Mr. Obama is said to be considering, would be a step in that direction but probably not a big enough one. Without stronger U.S. measures, the most likely outcome is the fragmentation of Syria into warring fiefdoms, with some turf controlled by Iran and some by al-Qaeda.

What’s needed is what the opposition has repeatedly requested: a no-fly zone in parts of Syria, or other measures — such as attacks with missiles and stealth bombers — to ground the Syrian air force. Yes, such measures would have to be taken without a United Nations resolution, and they would upset Mr. Putin. But if Mr. Obama continues to pursue a policy of awaiting U.N. consensus and deferring to Russia, the result will be more crossings of his red line — and grave damage to U.S. interests.