BS
They are another gang like the Mexican gangs, only maybe a little bigger and more bloodthirsty.
The first modern army to confront them will leave nothing but dead bodies in their wake.
Presumably with artillery and an air force. Remember - I wrote light infantry, and that's just what I mean. Equipped with technicals, rifles, mortars and RPG's, these people killed 5,000 GI's. That's a far bigger number than the much-vaunted Taliban from supposedly indomitable Afghanistan inflicted on US forces. And that's reflected in their results. Unlike their Afghan counterparts, ISIS have taken entire cities from Iraq and Syria despite only having a fraction of the manpower and resources of the sitting governments.
Comparing ISIS with Mexican gangs is just risible. When was the last time a Mexican gang took a city from the government? What's truly impressive about these people is that they have gone toe-to-toe with a "modern army" in situations like Fallujah with severe disadvantages (i.e. no on-call tube artillery, no live battlefield intel from drones and JSTARS, no air support on demand, no food and ammo resupply on request) and not staged a mass surrender in the initial hours of the hell storm unleashed upon them. Don't mistake atrocity for fear of battle. Like medieval combatants, these people are neither afraid to die nor squeamish about killing.