BEIRUT (AP) — A month after Russia, Turkey and Iran came together in a last-ditch effort to avert a potentially catastrophic Syrian government offensive in Idlib, they appear to have succeeded in creating a buffer zone around the northern rebel-held province, defusing tensions in a major flashpoint area.

The deal has for now averted a government offensive on the last major opposition stronghold in Syria, where tens of thousands of militants, including foreign jihadis, live alongside 3 million civilians and opposition fighters. But if the truce falls apart, the fighting could cause massive disp...