A plan to avert an offensive by the Assad government against the last major rebel-held area in Syria stumbled Monday when the main extremist group failed to leave a proposed buffer zone as part of a deal to demilitarize the region.

However, Russia may still prevent Syria’s President Bashar al-Assad from launching an assault to retake Idlib, a province in northwestern Syria, as Moscow now appears eager to avoid a fight and instead find a political settlement to end the Syrian war and cement its growing role as a power broker...