ISTANBUL—Ever since the Syrian revolution began in 2011, President Bashar al-Assad tried to cast it as a religious conflict with radical Sunni Islam in which he would wear the mantle of protector of the country’s numerous minorities.

The plan has worked to a great degree, with Mr. Assad’s own Alawite community as well as Shiites, Christians, Druse and, initially, even the Kurds, backing him against a predominantly Sunni rebellion that has become progressively more bloody and sectarian.

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