Jihad is a precept of Divine institution. Its performance by certain individuals may dispense others from it. We Malikis [one of the four schools of Muslim Jurisprudence] maintain that it is preferable not to begin hostilities with the enemy before having invited the latter to embrace the religion of Allah except where the enemy attacks first. They have the alternative of either converting to Islam or paying the poll tax (jizzya), short of which war will be declared against them. The jizya can only be accepted from them if they occupy a territory where our laws can be enforced. If they are out of our reach, the jizya cannot be accepted from them unless they come within our territory. Otherwise we will make war against them....
It is incumbent upon us to fight the enemy without inquiring as to whether we shall be under the command of a pious or depraved leader.
It is not prohibited to kill white non-Arabs who have been taken prisoner. But no one can be executed after having been granted the amân (protection). The promises made to them must not be broken. Women and children must not be executed and the killing of monks and rabbis must be avoided unless they have taken part in battle. Women also may be executed if they have participated in the fighting. The amân granted by the humblest Muslim must be recognized by other [Muslims]. Women and young children can also grant the amân when they are aware of' its significance. However, according to another opinion, it is only valid if confirmed by the imam (spiritual leader). The imâm will retain a fifth of the booty captured by the Muslims in the course of warfare and he will share the remaining four fifths among the soldiers of the army. Preferably, the apportioning will take place on enemy ground (p. 163).
Ibn Abi Zayd al-Qayrawani (d. 966)
In the Muslim community, the holy war is a religious duty, because of the universalism of the (Muslim) mission and (the obligation to) convert everybody to Islam either by persuasion or by force. Therefore, caliphate and royal authority are united (in Islam), so that the person in charge can devote the available strength to both of them (religion and politics) at the same time.
The other religious groups did not have a universal mission, and the holy war was not a religious duty to them, save only for purposes of defense. It has thus come about that the person in charge of religious affairs (in other religious groups) is not concerned with power politics at all. (Among them) royal authority comes to those who have it, by accident and in some way that has nothing to do with religion. It comes to them as the necessary result of group feeling, which by its very nature seeks to obtain royal authority, as we have mentioned before, and not because they are under obligation to gain power over other nations, as is the case with Islam. They are merely required to establish their religion among their own (people).
Thereafter, there were dissensions among the Christians with regard to their religion and to Christology. They split into groups and sects, which secured the support of the various Christian rulers against each other. At different times there appeared different sects. Finally, these sects crystallized into three groups, which constitute the (Christian) sects. Others have no significance. These are the Melchites, the Jacobites, and the Nestorians. We do not think that we should blacken the pages of this book with discussion of their dogmas of unbelief. In general, they are well known. All of them are unbelief. This is clearly stated in the noble Qur'an. (To) discuss or argue those things with them is not up to us. It is (for them to choose between) conversion to Islam, payment of the poll tax, or death (1: 480).
Ibn Khaldun (d. 1406)
Sources: Bat Ye'or, The Dhimmi. Rutherford: Fairleigh Dickinson University Press, 1985.