It still does not register with me how 15/20 thousand ISIS (mostly untrained) volunteers can take over such large swathes of Syria and Iraq despite a trained Iraqi and Syrian army, anti ISIS volunteers, logistical and air support from various countries.
Its not all that bemusing considering the fragmented and complicated micro-conflicts that are happening in those two nations. This isn't a war involving two groups of sides, but rather half a dozen or so factions, each with their own personal goals, each pretty much taking it in turns to transiently align themselves to one another, only to inevitably turn on one another when their personal goals come at odds.
In addition to receiving heaps of funding from Saudi and Qatari businessmen, ISIS have prudently capitalised on several factors which has given them an edge. Firstly, the sympathy of many Iraqis and Syrians who've decided that to spite their respective governments, they'd embrace IS in order to free themselves from the central governments of Baghdad and Damascus, it explains why they were able to seamlessly sweep into Mosul with no resistance and lay claim to the place. Of course, the locals are feeling a tad bit regretful and silly about it now that IS are essentially closing down universities and sticking heads up on pikes, but granted they're so embedded now its a little too late.
Another factor, and this applies more to Syria, has been the chaotic handover of weapons to the Syrian Rebel forces, facilitated by the Gulf Arab states, Turkey and the US. This was clearly a very irresponsible and short-sighted initiative considering that the most effective and dominant of the Syrian rebel forces were the most extreme elements linked to the likes of Al Nusra and IS, hence its hardly of any surprise that weapons perhaps intended for so-called 'moderates', had eventually found their way into the hands of those committing atrocities under the black flag of IS.
Finally, we musn't ignore the hugely successful recruitment drive ISIS have been able to cultivate in recent months. Bored Mujahadeens from Chechnya and North Africa have found a perfect calling in Syria and Iraq to satisfy their militant urges for martydrom, whereas hundreds, if not thousands of disillusioned British, French and American Muslims have been brainwashed into heeding their call, most likely because they feel marginalised in their respective societies.