The success of IS and other militant and rebel groups can be explained by an Arab historian.
To understand this phenomenon, we need to turn back to the 14th century Arab historian Ibn Khaldun, who observed the frequency of smaller, poorer groups defeating larger, better organized states. Khaldun posited that the cause of this phenomenon was a concept he termed asabiyyah. According to Khaldun, groups on the margins of society including tribes and today’s militants possess a strong sense of asabiyyah, which translates loosely as “group feeling.” Groups possessing strong asabiyyah have a strong sense of cohesion and solidarity and the individuals who made up these societies feel tightly bound to the group’s goals. On the other hand, groups that become more civilized and wealthy have a weaker sense of asabiyyah and become more lax and less coordinated because of the growth of wealth and individualism.
Thus, groups with a strong sense of solidarity and purpose frequently take their societies to great heights, but upon doing so, loose the cohesion and ability to maintain their power. As a result, they are frequently overwhelmed by newer groups possessing their original characteristics. These newer groups often see an opportunity to engage in a level of violence to achieve their goals unmatched by more established states. This explains why well-established, rich countries often feel overwhelmed in confronting small, militant groups despite the resources they have in doing so. Imagine how difficult it is then for weaker states with almost no sense of nationhood to confront militants and rebels.