European leaders signaled they would discuss lifting a ban on arming Syrian opposition fighters, as the U.S. approved the deployment of air-defense systems to Turkey and some 400 American troops to man them. The U.S. deployment would significantly expand the Pentagon's role along Syria's borders.

In Europe, the U.K. and France have led the charge to relax an arms embargo that prevents weapons being sent to opponents of Syrian President Bashar al-Assad. This week, most EU member states followed Paris and London's lead in formally...