Here's the latest news from Syria, and it does not bode well for the US policy of supporting those illusory "moderate" anti-Assad rebels.
Two CIA-approved rebel leaders, Jamal Marouf and Muhammad Hallak, have been losing ground and fighters to both al-Nusra and ISIS/Islamic State. When "moderate" rebels join the so-called extremist groups, presumably they take their US weapons and training with them. Thus far the American bombing campaign has accomplished little beyond being an effective recruiting tool for the very extremists it is supposedly targeting.
Furthermore, it shouldn't come as a surprise that al-Nusra and Islamic State can work together. They are united by their goal of overthrowing the Alawite regime in Damascus. They are also united in their determination to prevent post-Assad Syria from falling into the hands of a US-controlled puppet government, which is an inevitability were the so-call moderates ever to prevail.
In short, American policy seems designed to grind what's left of Syria into the same state of chaos as prevails in Iraq and Libya. The relevant question should be; who benefits from this?