The agreement that Secretary of State John Kerry has negotiated with Russia to conduct joint air operations against terrorists, in return for a cease-fire that allows for the delivery of humanitarian aid, has brought to the surface a long-simmering divide over the wisdom of entering a military alliance with Moscow, even for the limited goal of attacking the Islamic State.

Kerry’s plan calls for the creation of a “joint implementation center,” in neighboring Jordan, where U.S. and Russian officers would sit side-by-side to share intelligence and coordinate airstrikes against the Islamic State and other terrorist targets in Syria.