Syrian President Bashar al-Assad with his wife Asma. (Hassene Dridi -- Associated Press)

It's been a bad few days for Syrian leader Bashar al-Assad, as world leaders seem to converge on a new consensus that his days really are numbered and that the fall of his regime is imminent. Thursday, Russia's deputy foreign minister acknowledged, for the first time, that Assad might fall, and the rebels might win, though he emphasized that this possibility was "absolutely unacceptable" to Russia. The secretary-general of NATO piled on, calling the regime's collapse "only a matter of time." He was quickly followed by the finance minister of neighboring Iraq, who called it a matter of "weeks." Yes, it's just the finance minister, but his comments, on the sidelines of an IMF meeting in Jordan, hint that he may be expressing the Iraqi government's view. On Wednesday, President Obama said the United States would recognize a Syrian opposition group as the country's legitimate representation," which may have been motivated in part by a growing sense that Assad's fall was inevitable so the United States should start building ties with the group that might replace him.

So what led all these world leaders, 21 months into a war that has killed thousands, to all of a sudden come around to the idea that Assad is likely to fall? 

Rebel advances are accelerating, maybe overwhelmingly. The simplest explanation is often the best one: The rebels look more and more likely to win. Though the Syrian military still holds most of the firepower, rebels seem to be gaining both physical and ideological momentum. It's a self-reinforcing cycle: The more military bases the rebels take, the more arms they pick up, and they easier the next base is to topple. Same goes for all-important military defections. If you're a Syrian lieutenant, then the more of your military buddies walk away from their posts, the more likely you probably are to join them. These rebel gains are increasingly evident, and a few watershed moments – shooting down military helicopters, for example – have made them tough to ignore.

Assad's regime was probably never going to recover legitimacy at this point. There is a precedent for an authoritarian government using force against its own people and then later regaining domestic legitimacy. China's bloody 1989 crackdown in the areas around Tiananmen Square, for example, was over so quickly – and followed by such tremendous economic growth – that the Communist Party has largely recovered. And Assad's own father, Hafez al-Assad, killed thousands in a 1982 crackdown on an uprising in the city of Hama, yet still ruled until his death in 2000. But Bashar al-Assad's violence has now dragged on for almost two years, touched nearly every corner of the country, and been widely documented in the process. Even if Assad puts down the rebels, at this point he was probably never going to fully reestablish rule, which means he would never again be a fully useful ally for Russia, his most important patron.

If he's going to fall, better for it to happen soon, before the war gets any worse. The regime has made some disturbing moves in the last two weeks that have raised fears it might escalate its violence. It reportedly fired a handful of Scud missiles at rebel groups; the missiles are inaccurate but long-range and chemical-capable, making them good tools for deterring a foreign invasion. The Syrian army has also been re-shuffling its large chemical weapons stockpile. It's possible this was just a security precaution, but even if the regime never uses the weapons, the odds of rebels getting a hold of them increase as the war drags on. The threat of anyone using chemical weapons could be enough to nudge world leaders toward encouraging a rapid end to the conflict, even if it means that Moscow might see its favored side lose.