A mystery that lends itself to conspiracy theories because the story doesn’t add up:
How did ISIS rise within a few years to have the military might & geographical scope of a powerful nation-state taking on at least four major military powers at once along with organizing a terrorist auxiliary force operating in Paris, San Bernardino, Dhaka, Istanbul, Baghdad, & with an outpost stronghold in Libya? Who the hell is arming them & why aren’t the armament supply routes blocked?
Some of those militaries operating against ISIS stopped the German army in WWII & they can’t stop ISIS, supposedly a paramilitary force?
Don’t answer the question with conspiracy theories. Even though other explanations are not forthcoming.