Vali Nasr is professor at Tufts University, a senior fellow at the Brookings Institution and the author of “The Shia Revival: How Conflicts Within Islam Will Shape the Future.”

THE Arab Spring is a hopeful chapter in Middle Eastern politics, but the region’s history points to darker outcomes. There are no recent examples of extended power-sharing or peaceful transitions to democracy in the Arab world. When dictatorships crack, budding democracies are more than likely to be greeted by violence and paralysis. Sectarian divisions — the bane of many Middle Eastern societies — will then emerge, as competing groups settle old scores and vie for power.

Syria today stands at the edge of such an upheaval. The brutality of Bashar al-Assad’s regime is opening a dangerous fissure between the Alawite minority, which rules the country, and the majority Sunni population. After Mr. Assad’s butchery in the largely Sunni city of Hama on July 31, on the eve of the holy month of Ramadan, the Muslim Brotherhood, a Sunni group, accused the regime of conducting “a war of sectarian cleansing.” It is now clear that Mr. Assad’s strategy is to divide the opposition by stoking sectarian conflict.

Sunni extremists have reacted by attacking Alawite families and businesses, especially in towns near the Iraq border. The potential for a broader clash between Alawites and Sunnis is clear, and it would probably not be confined to Syria. Instead, it would carry a risk of setting off a regional dynamic that could overwhelm the hopeful narrative of the Arab Spring itself, replacing it with a much aggravated power struggle along sectarian lines.