MOHAMED AZAKIR / REUTERS Forces loyal to Syria's President Bashar Assad carry the national flag as they ride on motorcycles in al-Qusayr, after the Syrian army took control from rebel fighters, on June 5, 2013

The battle of al-Qusayr, the border town that Syrian government forces wrested from rebels on Wednesday after two bloody weeks of combat, carries significance beyond its position as a crucial crossroads and battered symbol of the staying power of Syrian President Bashar Assad. Al-Qusayr also marks the point at which Syria’s civil war indisputably flared beyond the borders of a single, riven nation and began to fulfill dire prophecies that the bloodletting there would produce a broader conflict that might engulf the whole Middle East on the lines of Islam’s main sects: Sunni against Shi‘ite.

Al-Qusayr changed hands with the self-acknowledged help of Hizballah, the Shi‘ite fighting force based in Lebanon. Its announced deployment into Syria a week ago was answered by calls from prominent Sunni clerics to engage the rival sect on the field of battle. Thus has a contest that began more than two years ago with peaceful marches against an Arab autocrat been redefined as a bloody conflict rooted in identity and spilling beyond Syrian national borders.

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On one side stands Assad, a member of the Alawite sect — usually described as heterodox, but in the binary world of armed conflict is firmly placed on the Shi‘ite side of the ledger. In this case the alignment of faith is underscored by politics: Assad’s Syria long ago forged an iron alliance with Iran, the titan of the Shi‘ite world. Together, the two governments nurture Hizballah, the militia that empowered much of Lebanon’s historically impoverished Shi‘ite population, but which supposedly exists to challenge Israel.

Its enemies in Syria, however, are the Sunni fighters who dominate the rebel forces fighting to topple Assad. The motley groups run a gamut. Some are nationalists who answered Assad’s violent response to the Arab Spring protests by resorting to arms themselves, and happen to be Sunni, the majority sect in Syria as among Muslims globally. Other fighters assemble under religious flags of varying intensity, the most intense being the Jabhat al-Nusra faction formally aligned with al-Qaeda.

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The strict orthodoxy enforced by Sunni radicals condemns any doctrine that deviates from their own, and sees the Shi‘ite as “more infidel than Jews or Christians,” in the words of Sheik Yusuf al-Qaradawi, the Sunni cleric who on Saturday summoned all Sunnis trained for fighting to assemble in Syria. “Iran is pushing forward arms and men, so why do we stand idle?” asked al-Qaradawi, an Egyptian speaking from his base in Qatar, where he has a wide following on al-Jazeera, the Qatar-based TV news channel. Playing on the Arabic translation of Hizballah, or Party of God, the cleric said, “The leader of the party of the Satan comes to fight the Sunnis … Now we know what the Iranians want … They want continued massacres to kill Sunnis.”

Inside Lebanon, senior clerics in the Muslim Scholars Association, an assembly of Sunni clergy from across the religiously complex nation, reached back into history, declaring that the failure to rally around the battle to keep al-Qusayr in rebel hands would be punished by “the spreading of the Safawi project.” The reference was not arcane to the devout: it was the Safavid dynasty that converted Iran from a Sunni nation to Shiism. The two sects are nominally divided on the question of who was justly entitled to succeed the Prophet Muhammad as leader of Muslim community after his death: family or friends? Sunnis believe it was his closest followers. Shi‘ites believe the mantle fell to the husband of his favorite daughter. But over centuries the competing narratives have produced bitter judgments. A 2012 Pew Poll found 40% of Arab Sunnis do not believe Shi‘ites qualify as Muslims.

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In stable times, the differences in faith can be papered over — first by the overarching principles shared by all Muslim believers, but also by a strong national identity, something that bound Iraqis together for a long time before the outrages committed by extremists provoked the rampant sectarian bloodletting there beginning in 2005. “When states are weak, sectarianism rises,” the Lebanese religious scholar Hani Fahs says in a new Brookings report, The New Sectarianism: The Arab Uprisings and the Rebirth of the Shi‘a-Sunni Divide. “People return to their primary identities. And the more religiosity in a society, the more the state is weak.”

So while Assad’s side racks up battlefield wins inside Syria with the help of armaments from Russia and fighters from Iran, the more significant action may be occurring outside its borders. While Hizballah pushed forces into al-Qusayr, 16 shells and rockets landed on May 31. On Friday inside Lebanon, in Baalbek, a Bekaa Valley city regarded as a Hizballah stronghold, gunmen fired on a Shi‘ite shrine. In a Shi‘ite neighborhood of Beirut, young men attacked a car carrying the Sunni cleric who heads the Muslim Scholars Association. In Tripoli, in Lebanon’s north, the death toll stood at eight in fighting between Sunni and Alawite residents, a month after 31 were killed in another spasm of violence mirroring the war to the north.

On the BBC, the commander of the Free Syrian Army, the umbrella group that claims to represent most of Syria’s rebel forces, said Hizballah should not expect the fighting to respect Lebanon’s borders if Hizballah does not respect Syria’s. “Hizballah fighters are invading Syrian territory,” said the commander, Salim Idris. “And when they continue to do that and the Lebanese authorities don’t take any action to stop them coming to Syria, I think we are allowed to fight Hizballah fighters inside [Lebanese] territory.” Fitting, then, that al-Qusayr was a battle for a border town.

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