Insurgents seized one of the last Syrian government positions in the northern province of Idlib on Thursday night, in the latest in a series of recent gains by a newly effective opposition coalition that includes the Syrian branch of Al Qaeda as well as other Islamist factions and a few nationalist groups backed by the United States. The government forces’ quick retreat from the small city of Ariha left insurgents celebrating in the streets and made it appear increasingly unlikely that forces loyal to President Bashar al-Assad would retake the provincial capital, also called Idlib, as officials vowed to do when it fell to insurgents two months ago. The government is also facing advances by the Islamic State, which seized the central desert city of Palmyra last week. Taking Ariha puts the insurgents a step closer to the government’s coastal strongholds and the plains around the city of Hama.