Photograph by Moises Saman/Magnum.

All wars are marked by historic watersheds, and in Syria there have been many, but the past four weeks have supplied a dizzying flurry of them, surely more than at any time since the uprising against Assad’s rule began in March, 2011.

If there were any lingering doubts as to the sectarian nature of Syria’s civil war, the defection, last Sunday, of its prime minister, Riyad Hijab, to the opposition Free Syrian Army, should have ended them. Hijab, like all the other recent high-level defectors, is a Sunni Muslim. An estimated seventy-five per cent of Syrians are Sunnis, and Syria’s rebels are overwhelmingly Sunni. The government led by Bashar al-Assad, meanwhile, is dominated by the Alawites, a minority Shiite sect, representing some fourteen per cent of the Syrian population. (About ten per cent of Syrians are Christians; they are seen as neutral in the conflict.)

Hijab was not alone. The violence in Syria has undergone a dramatic upsurge in recent weeks, and the defections of senior officials have added to the sense that the regime’s demise is quickening. After months of gradually escalating violence, but relatively few betrayals at the senior levels of government, Manaf Tlass, a prominent brigadier general and onetime personal friend of Assad’s, vanished on July 6th, and helped kick things into a new gear. That defection was followed by that of the ambassador to Iraq, Nawaf al-Fares, and then by those of two more diplomats, the envoys to the United Arab Emirates and Cyprus, on July 25th.

Augmenting the sense of gathering apocalypse was the spectacular bombing in Damascus, on July 18th, of a meeting of Syria’s national-security team, in which four of Assad’s closest aides, including his deputy defense minister and brother-in-law, Assef Shawkat, were killed. The bombing coincided with unprecedented rebel assaults on Damascus and the city of Aleppo. In both cities, which were relative oases of calm until that point, fierce fighting has continued ever since. After successfully crushing the rebel strongholds in the capital for now, Assad’s forces have turned their attentions on Aleppo. In recent days, several thousand troops, accompanied by tanks and combat aircraft, have begun a counteroffensive to wrest back control of the city. Last Thursday, in another sign of descent, Kofi Annan announced that he was resigning as the United Nations’ mediator.

The aura of invincibility that had sustained the regime has been deeply undermined. Assad himself has abandoned the pretense of his own inviolability, and has reappeared only virtually, as a disembodied voice on a handful of broadcasts, or, as he did on Tuesday, on a prerecorded video clip showing him conducting a formal meeting with Saeed Jalili, the Iranian national-security chief—his first television appearance in two weeks. (For those that say that Assad’s vanishing shows that his regime is finished, however, it’s worth remembering that Saddam Hussein managed to hang on in a similar virtual fashion, rarely ever being seen in public, following the first Gulf War, in 1991, and until his ouster in the American-led invasion, in 2003.)

Before the current stream of defections, it was an article of faith among analysts that the Syrian rebellion against Assad was hampered by the lack of any visible fractures at the top. In comparison to Libya—where senior Qaddafi officials began jumping ship almost immediately, providing the opposition in that country with a cast of characters that was sufficiently capable to muster a National Transitional Council—Syria’s efforts to forge a rebel leadership had been lackluster and inadequate. That finally has begun to change—although, again, the sectarian hue of the regime, and of the rebel opposition, is being further underscored by the defections.

No senior Alawites have come over to the Free Syrian Army, and, one suspects, that at this point there may be few forthcoming. What to think, then, of the defecting Sunnis? They are rank opportunists, one would guess; individuals who, until now, from their official positions of power and influence within the regime, were happy to put aside their moral qualms even as the body count in Syria has neared the twenty-thousand mark. Perhaps for some it is less about opportunism than security; it may have reached the point inside official ranks that anyone who is a Sunni is now suspect, and life for them has become untenable.

Whichever the case, it’s clear enough that whatever it was before, Syria’s conflict is being fought along sectarian lines. The same holds true for the widening regional links being formed. Just as the Shiite-led Islamic Republic of Iran, and its Lebanese Shiite proxy, Hezbollah, are among Assad’s closest and most steadfast allies, the Sunni Muslim states of Saudi Arabia, Qatar, and Turkey are the main backers of the Syrian rebels. There is, of course, a geostrategic dimension to this as well: behind the Shiites stand Russia and China; behind the Sunnis stand the United States.

What does all of this side-taking mean? It means that whatever else happens inside Syria—and it will be ugly, for sure, and involve a great deal more killing—there is also an international showdown taking place. In the end, Syria may only turn out to be a skirmish rather than a wider conflict, as some fear. But it seems possible that Syria is destined to be a historic turning point. Whichever way it goes, post-Syria, we may well be speaking openly about a new Cold War, with the international battle lines drawn roughly as they are today around Syria, and with new proxy conflicts yet to come.

Read more of Jon Lee Anderson’s dispatches from Syria.