Growing numbers of foreign Sunni extremist fighters are battling not just to rid Syria of Mr. Assad, but to religiously cleanse it. As a result, many Syrian Christians now fear that their fate will mirror that of Iraqi Christians, who were largely forced out of Iraq by war and sectarian terrorism. The city of Homs was once home to 80,000 Christians; there are now reportedly fewer than 400.

Three vetoes by Russia and China have blocked attempts by the United Nations Security Council to hold the Syrian government accountable for its crimes. But those who have opted for a proxy war in Syria and who are now financing the rebels cannot avoid responsibility for what comes next.

Governments that have publicly committed themselves to helping end Syria’s misery, including the United States, must immediately do two things to help prevent a violent backlash against Alawites and other minorities. First, they must impress upon the newly united Syrian opposition that support depends on strict adherence to international humanitarian law. Armed groups who advocate fracturing Syria along sectarian or regional lines should be denied funds; there should be absolutely no aid for rebel groups who target Alawites and other minorities for reprisals or who commit war crimes.