Considering the rapid expansion of the Muslim empire during the reigns of Muhammad's political successors, the caliphs, it should come as no surprise that medieval scholars would have much to say on the topic of armed jihad. And these discussions were by no means monolithic: much like the Jewish rabbis, Muslim scholars developed a rich legal tradition marked by considerable diversity.

We shall now turn our attention to the medieval and modern Islamic legal discourse on jihad. What follows is not intended to be an exhaustive account of Muslim scholars’ shared and conflicting rules of armed jihad but rather a survey of the principles most relevant to the subject of the present book. Accordingly, as both al-Qaeda and ISIS represent radical Sunni organizations, our focus here will be Sunni (as opposed to Shi'ite) Islamic law.

JUSTIFYING JIHAD

Medieval scholars delineated two major forms of armed jihad, one defensive, the other aggressive:

Although Muslim scholars conceptualized aggressive jihad as a means of promulgating Islam (typically through influence and incentives, not mandatory conversion), there was nonetheless a widespread belief that such a jihad was necessary to protect and preserve Muslim communities. Thus, while various scholars saw the unbelief of neighboring non-Muslims as sufficient grounds for declaring war on them, a popular view was that even aggressive jihad was ultimately defensive in nature. The assumed “state of war” in Arabia was not unlike the state of affairs throughout much of the premodern world: a premodern country, having no fixed territorial borders, was often as safe as it was aggressive.