A fragile truce was holding on Syria's southern front on Tuesday after Jabhat Al Nusra killed a senior commander in a western-backed rebel group, accusing his faction of being sleeper agents for ISIL. Mousab Ali Qarfan, who also went by the name Mousab Zaytouneh, was a leading figure in the powerful Shuhada Al Yarmouk Brigades. He was killed by the Al Qaeda-affiliated Nusra in Sahem El Golan, along with three other fighters from his group, on Monday. The Yarmouk Brigades are part of the western-affiliated rebel alliance, still commonly referred to as the Free Syrian Army (FSA), which have been...