BEIRUT — In the ever-shifting landscape of the Syrian civil war, the line between allies and enemies is rarely clear. It was further muddied this week when the U.S.-led coalition revealed plans for a 30,000-strong security force to police Syria’s northeast borders with Turkey and Iran. The plan instantly enraged Turkey, a NATO ally of the U.S., because it would rely heavily on Kurdish fighters who are viewed as terrorists by the Turks. “Is the duty of protecting NATO borders left to terror groups?