Jihadi veterans known collectively as the Khorasan group, which have been targeted in two waves of airstrikes by U.S. warplanes, are trying to broker an alarming merger between militant archrivals the Islamic State andJabhat al Nusra, the official Syrian branch of al Qaeda.

The merger, if it comes off, would have major ramifications for the West. It would reshape an already complex battlefield in Syria, shift forces further against Western interests, and worsen the prospects for survival of the dwindling and squabbling bands of moderate rebels the U.S. is backing and is planning to train.

“Khorasan sees its role now as securing an end to the internal conflict between Islamic State and al Nusra,” says a senior rebel source. The first results are already being seen on the ground in northern Syria with a coordinated attack on two rebel militias favored by Washington. […]