But that's because I read about the Anabaptists and the Siege of Muenster.
See, in the early 1500s, the Protestants were fired-up and eager to throw off the shackles of the Catholic church. Some of them crossed the line from "eager" to "wacko."
One of these groups was the Anabaptists, who believed that baptism was only valid if it happened to adults, because a squawling infant has no ability to consent to the grace of God. That's familiar territory for anyone from the American South. But the ANAbaptists also believed that Jesus Christ was going to come down from heaven any year, anymonth, any day soon, and smite all the unbelievers. Like the Catholics. And the other Protestants. And anyone who disagreed with them. They were very much an "apocalypse now please" group.
And in the city of Muenster, they slowly overtook all positions of power, and expelled all the non-Anabaptists, based on the charismatic leadership of a visionary prophet.
The city was then besieged by its legal ruler, the Prince Bishop of Muenster, and it went about as well for the people inside as the Waco siege, only with slow starvation instead of fire.
Now, what you had in the Islamic State was very similar ingredients. Messianic charismatic leadership in a time of religious and political turmoil; attempts to win converts from outside and bring them in, ethno-religious conflicts exploding after years of smoldering, a defiant stand against the decadence of the established world order, strict religious rules within the group, and the promise of Heaven's reward. Plus the expectation of righteous armageddon within a decade. (I get the feeling that Christianity and Islam have the same propensity for violent messianic apocalyptic movements. Something about monotheism does that to people.)
The difference between ISIS and the Anabaptists is that the Islamic State actually had weapons and the ability to fight against a siege, whereas the Anabaptists had very little after they repulsed two attacks and began to run out of food.
Then again, from the vantage point of 2018 ISIS had about as much success as the Anabaptists of Muenster. Sometimes I wonder if the whole thing was overblown.