Russia and Iran have rushed to the aid of an embattled ally as the government of Syrian President Bashar Assad announced a counteroffensive against rebel groups that have scored stunning advances in the past 48 hours. Russian warplanes hit rebel-held targets in the northwest region of Idlib on Sunday as Iran’s foreign minister made an emergency trip to Damascus to demonstrate support for the Assad regime. Syrian jets also hit rebel-held areas of the cities of Aleppo and Idlib which have fallen to the insurgents’ lightning advance. The moves were a sign of the deep shock delivered to Mr. Assad,...