The US decision to open up its attacks on ISIS in Syria earlier this week came with a decision to attack another faction, which they dubbed Khorasan.
The term Khorasan was basically a US invention, and what is being attacked in that case is Jabhat al-Nusra, an al-Qaeda affiliate. The US strikes are pushing Nusra to forge new ties with ISIS.
ISIS has long been at odds with the other rebel factions, including Nusra, and had been in direct conflict with them off and on. The US insinuation of itself into the war, however, has given both sides a very high-profile common enemy.
Way back when, ISIS (then still al-Qaeda in Iraq) announced a “merger” with al-Nusra, which brought them several recruits, but also split Nusra in half, and fueled a split between ISIS and al-Qaeda in the process. Increasingly, that division seems like it could be wiped out by the US war.