It didn’t help their cause that they were and are more than willing to kill their Sunni co-religionists, people who if they’d been the least little bit nice to would have been eager for their help to throw off the Shiite-run central government in Baghdad. Instead, the few Sunni allies they made are increasingly being driven to fight against them, the survivors of the Sunnis they killed are even considering making common cause with the Shiites and even the Kurds to drive out ISIS. The Shiites and Kurds have themselves shelved, at least temporarily, their differences in order to fight ISIS, and with the US arming the Kurds and providing a few airstrikes here and there, they are succeeding.
Far from being the Machiavellian world-beaters people with various agendas proclaim them to be, the people behind ISIS are typical bloodthirsty money-grubbing bullies, which means that they vastly prefer soft targets, the softer the better.
The entire history of ISIS has been the continuing search for ever-softer targets. They got their start in Jordan in 2000 and 2001, but soon got kicked out by Jordanian security forces which were more than a match for them. They would have likely faded away at that point, but for Bush and Cheney invading and toppling Iraq’s Sunni-dominated version of state-sponsored, Socialist-flavored capitalism under the pretext of avenging 9/11 and hunting up nonexistent WMD. They then made some initial moves into the Sunni portions of Iraq, but stopped when the going got rough and wound up forsaking the Sunni cities for the remote and sparsely-populated parts of Anbar province, where they stayed hunkered down until 2012. The Syrian Civil War gave them their next shot at establishing a caliphate in something other than empty and forsaken terrain nobody else wanted, and just as in Iraq right now, it seemed that they were going to succeed. But even with Saudi and other money pouring into their coffers, they couldn’t get the job done, and in large part for the same reason why they can’t get the job done now in Iraq: They didn’t get along well with the existing local Sunni groups, as they saw themselves as the cocks of the walk. So they took the money they’d saved from the donations of various rich Sunni backers in Saudi Arabia and Kuwait and used it to fund a second attempt at carving out a caliphate in Iraq — an attempt which is now floundering and collapsing. The only reasons they haven’t already been driven out totally are their money (which while substantial isn’t enough to form an actual government) and the inability of Shia and Sunni to form long-term working alliances with each other (though both groups have done so with the Kurds, which is astonishing in itself: It wasn’t that long ago that the Shiites under Maliki were trying to starve the Kurds of oil revenue, and the Sunnis didn’t like them all that much either).
What happens next? Well, the groundwork’s already been laid for that. As Gary Brecher points out, they become the favorite pretext for Israel to continue going after Hamas and Gaza:
Look at the Golan Heights, occupied by Israel. Look east, and notice that right under the heavy artillery of the IDF lie the lowlands occupied by Sunni jihadis — sitting ducks for air or art’y strikes — yet who have never once been hit by the IDF. All this time, as the IDF’s planes and tubes blasted Hezbollah every chance they got, they’ve looked down onto the plains east of Golan and gazed upon a patchwork quilt of Sunni thug militias with the fond eye of a poisoner watching his petri dishes of Ricin bubble and fizz.
Israeli military intelligence has a thousand and one uses for a group like I.S.I.S., and now that Gaza has exploded again, they’ve found a new way to use their little Sunni thug buddies: As a way to smear Hamas, a more serious, sane Sunni militia. And that — Jeez, I can’t believe I even have to explain this it’s so fucking obvious — that, friends, is why you’re getting all this “I.S.I.S. in Gaza!” crap.
There you have it.