Meantime, Russia and China have made clear they'll veto Western efforts to give the mandate teeth by imposing sanctions if Assad stonewalls. Without enforcement authority, the Annan mission is toast.
"I've been in constant touch with commanders on the ground," says Syrian journalist-in-exile Eiad Shurbaji, who now does Skype interviews daily from Washington. "As far as the [fighters] are concerned, they have gone beyond Kofi Annan's diplomacy. The real decisions won't be made in New York City, but on the ground or in Damascus."
Shurbaji was a rising young Syrian star, who owned a magazine and publishing house before the revolution; he had even interviewed Assad's wife, Asma - twice. When the uprising began, and he wrote about the regime's brutal crackdowns on civilians, he was arrested and beaten. He barely escaped alive to neighboring Jordan. (Three men who helped him escape were caught and shot.) His personal story shows why a peaceful rebellion turned violent under government repression.
Today, he says, Syrian rebels "feel they can only achieve their aims through battle." They view the Annan mission as a smokescreen that enables reluctant Western governments to delay any decision about whether to give military aid to the rebels.
That decision can no longer be postponed.
There are many understandable reasons that the Obama administration has been reluctant to help the rebels militarily. Still enmeshed in Afghanistan, barely out of Iraq, Americans don't want to get embroiled in another Mideast conflict. The Syrian opposition is splintered and disorganized, and no one knows who will take over after Assad.
U.S. officials hesitate to arm the opposition because of uncertainty about who will receive the weapons. The fighting is attracting foreign jihadis, even members of al-Qaeda, and the Muslim Brotherhood is the best-organized external opposition group.
Yet diplomacy, blocked by Moscow, has reached a dead end. The longer the United States hesitates to help the opposition, the more likely the war will devolve into a bloody sectarian conflict that produces an anti-American, possibly Islamist, government.
The only hope of avoiding that nightmare is by convincing Assad (and the Russians) that he has no chance to defeat the rebels and should retire to Moscow - and by encouraging a swift implosion of the regime.
Right now, the opposition is receiving (insufficient) funds for weapons, mainly from Saudi Arabia, Qatar, and other Gulf states. Those who give the funds have the greatest influence over where the money winds up. Already, many Syrian field commanders are complaining that Islamist fighters are favored over seculars. The United States has a small number of CIA officials in Turkey who are reportedly helping decide who should get the money inside Syria. However, Syrian activists say that presence is much too limited to make a difference.
The Damascus explosion creates a ripe opportunity for the West to change direction. A shift in U.S. policy could encourage more Syrian regime insiders to jump ship.
I asked journalist Shurbaji what the opposition needed now. He stressed: "Nobody wants American boots on the ground." But the rebels want concrete U.S. actions, not just speeches, that convince Assad (and Moscow) that Washington really seeks his ouster.
The opposition wants NATO to create, and patrol, no-fly zones inside Syria, along the Turkish border, which Syrian helicopters and tanks could not attack lest they be shot down.
If NATO resists taking to the air, there are other options.
Prime among them: Ensure that rebel commanders get the weapons they need, especially antitank and antiaircraft missiles. If the United States provides the funds, and intensifies contacts with rebel commanders, the weapons will be distributed more fairly, and reach more secular fighters.
As for fears that these weapons will reach the wrong hands, the rebels point out that Syrian tanks and helicopters are aging and that they don't need sophisticated weapons to shoot them down.
"The U.S. refusal to take a lead role," says Shurbaji, has convinced Assad that Washington doesn't really want him to leave. If the Americans referred Assad to the International Criminal Court on war crimes charges, that would have a big impact.
The longer this war lasts, the worse the likely outcome, including the risk that a desperate Assad will use chemical weapons against his own people. Now is the moment for Washington to shorten the conflict by giving the rebels stronger, concrete, backing. If the administration wants a say in the shape of a post-Assad government, it must catch up quickly with the realities on the ground.
E-mail Trudy Rubin at email@example.com.