Toby Matthiesen makes the case in the NYRB blog:
Since late May, pictures of Hezbollah militants standing amid the ruins of al-Qusayr, the former Syrian rebel stronghold, have offered dramatic evidence of the extent to which foreign Shia fighters are shifting the course of the Syrian war. To many observers, the Lebanese militia’s entry into the conflict has shown definitively that it has been a sectarian war from the outset. According to this view, Syria’s Alawite sect, to which the Assad clan and its security forces belong, is “quasi Shiite,” a fact which accounts for the government’s alliances to Iran and Hezbollah; while Syrian rebel forces are overwhelmingly dominated by the country’s aggrieved Sunni majority, now backed by the Sunni governments of Saudi Arabia, Qatar, and Turkey, along with various foreign Sunni jihadis.
But Bashar al-Assad is head of an ostensibly secular Baathist regime and many Shia think that Alawites are heretics. Why exactly is Hezbollah getting involved, and is this conflict really rooted in religion? The answer to both these questions may lie in a suburb of Damascus called Sayyida Zainab, the site of an important Shia shrine and since the 1970s a haven for foreign Shia activists and migrants in Syria. Today, Hezbollah forces, along with Iraqi Shia fighters, defend the suburb. Though the story of Sayyida Zainab is little known in the West, it may help explain why what began as a peaceful uprising against secular authoritarian rule in 2011 has increasingly become a war between Shia and Sunni that has engulfed much of the surrounding region.