Over the last few days, two U.S.-backed rebel groups in Syria have been fighting pitched battles in northern Aleppo. But rather than battling the Islamic State or Syrian loyalists, the rebels have instead been fighting among themselves. The skirmishing between the Marea operations room, a coalition of mainly Free Syrian Army units that holds positions against the Islamic State in northern Aleppo, and the Jaish al-Thuwar rebel group, which is a part of the Syrian Democratic Forces, threatens to undermine international efforts to utilize rebel factions to drive the Islamic State out of the area. To make matters worse, the local conflict has the potential to spread, enveloping the Kurdish People's Protection Units, better known by the acronym YPG....
