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As Assad’s army weakens, rebels and analysts warn, it is also becoming more extreme in its fight for survival. And two of the grimmest scenarios observers have long feared—that Islamic extremism could come to dominate the rebel fight, and that Assad could decide to attack with chemical weapons—now look more likely than ever to take hold. Many Sunni soldiers have left the military’s ranks, leaving behind a core of dedicated troops, most of them Alawites like Assad. Though the sect makes up a relatively small proportion of the Syrian population—around 12 percent—it has dominated the country’s political machine for decades, and Assad has painted himself as the protector of the Alawites, along with other minorities such as Christians, Druze, and...