ISIS has been around for several years, operating in Iraq, mostly in Tal Afar and Baghdad.
Al Nusra was the AQ group that established in Syria because the Free Syrian Army and its coalition of anti-Assad fighters were ineffective on the battlefield and didn’t allow the radical jihadists into their coalition.
Al Nusra is well connected to Zawahiri and his wealthy Qatari financiers. They also were well connected to the Libyans that now owned much of Qaddafi’s weapons. A marriage made in hell.
So the Libyans and Qatari’s puffed up Al Nusra that then expanded the fight against Assad.
ISIL saw the success of Al Nusra and decided to move its war machine from Iraq to Syria, where ISIS then tapped the Qatari money and Libyan weapons also, competing with Al Nusra.
That is why Zawahiri kicked them out of Al Nusra. ISIS then decided to go back to Iraq for some easy pickings (like robbing the Mosul banks) because the Iraqi army is not combat effective.
ISIS has made a major tactical blunder because its leadership does not see a larger picture and feels immune from attack. Much like Hitler invading Russia too soon.
Now ISIS has no safe haven and what ever they do in the future, they’ll do it walking and living in tents.
35,000 fighters with a couple of hundred thousand hangers-on does not a genuine, existential threat make.
The Saudis or Iran or Turkey could take care of the problem in a few weeks if they desired.
Going to war over a couple of beheadings is just plain dumb. What is the political origin of such rhetoric?
I think they're true believers. They're not just using religion as a cover for temporal ambitions. And you kind of have to be a true believer to mount any kind of war effort against the combined might of the infidel powers. In Muhammad's time, the technological gap between his tribesmen and the Byzantines to the West and Sassanids to the East was bridgeable via the capture or the hiring of artisans to build siege engines. Today's gap is far too large. You can't hire designers to build F-15's, petroleum refineries and cutting edge anti-aircraft missile batteries. The lead time is too long, and the human talent and the resources necessary out of their league, and even the financial league of the gulf state treasuries combined, unless they erase their lavish cradle-to-grave welfare systems.