A Hezbollah-led assault this week on the rebel-held town of Zabadani near Damascus appears consistent with an Iranian-backed plan for the Syrian regime to withdraw its forces to a more defendable line in the west of the country. For months, analysts have seen a pull back to a “rump state” as inevitable given the exhaustion of the Syrian army, the critical shortage of loyalist fighters, limits on Iranian resources, and territorial gains made this year by rebel forces in the north and south of the country. While that assessment hasn’t changed, President Bashar al-Assad’s regime so far is showing little...