Patricia Crone in openDemocracy:
[J]ust what is jihad?
Well, actually there are two kinds, depending on whether the Muslims are politically strong or weak. I shall start with the type associated with political strength, because that’s the normal type in Islamic history. I shall get to the second in connection with the question of modern relevance.
The normal type of jihad is missionary warfare. That’s how you’ll find it described in the classical law-books, from about 800 to about 1800. What the Quran has to say on the subject is a different question: the rules it presupposes seem to be a good deal more pacifist than those developed by the jurists and exegetes. But it is the work of the latter which came to form the sharia – the huge mass of precepts on which the public and private lives of Muslims were based (at least in theory), down to the coming of modernity, which still regulates their devotional lives today, and on which Islamists (or “fundamentalists”) would like once more to base the entire arena of public life.
The scholars said that jihad consisted in backing the call to Islam with violence, where necessary. It was “the forcible mission assisted by the unsheathed sword against wrongheaded people who arrogantly refuse to accept the plain truth after it has become clear”: thus a scholar who died in 1085. The idea was that God was the only ruler of the universe. Humans who refused to acknowledge this were in the nature of rebels, who had to be brought to heel. At the very least, they had to submit to God politically, by being brought under Muslim government. But ideally, they would submit to him in religious terms as well, by converting.