I have undertaken a study of military slavery for two reasons: to explain the rationale for this puzzling phenomenon and to examine an institution found only among Muslims.
Military slavery (as I define the term) refers not to any slaves in war, but only to those who fight as a result of conscious efforts by their masters to use them as soldiers. The slave is systematically acquired, trained in an organized way, and employed as a professional soldier.
A glance at slaves in warfare around the world shows that they filled auxiliary, support, and emergency roles nearly everywhere, but that they fought as professional soldiers almost only in Muslim countries. Within Muslim armies, many cases of military slavery existed, from the 9th century to the 19th, from Spain to Bengal, from Central Africa to Central Asia. I have located about one hundred and fifty dynasties in which slaves formed an important portion of the army. It therefore appears that most Muslim armies made use of this type of soldier, while non-Muslim armies almost never did.
An explanation for the existence of military slavery must take this fact into account. It will not suffice as an explanation to point out the advantages of using slaves as soldiers; for if military slavery can be understood by its benefits alone, why did non-Muslims hardly ever make use of them? An answer to the question "why did military slavery exist?" must also answer "why did it not exist outside the Muslim countries?" I shall approach these problems by assuming that Muslims had special needs which military slavery could fill; then I ask, what characteristics of Muslim armies called this system into being?
The answer lies perhaps in the fact that pre-1800 Muslim armies generally shared one overwhelmingly important feature in common: nearly all relied heavily on outsiders. By "outsiders" I mean aliens, social outcastes, religious minorities – persons who did not constitute the majority population of the country in question. In Muslim countries, very few soldiers were recruited from the agricultural or urban populations; Muslim rulers tended not to depend on their own subjects for military (or administrative) manpower. Instead, the great majority of soldiers came from the outside.
It is easy to note this pattern of recruitment but difficult to explain it. I have a tentative hypothesis for it to put forward; suggestions from readers are more than welcome.
Muslim farmers and city-dwellers tended not to make up the armies because they did not identify with their ruler and did not really respect his rule. The populace saw how territorial rulers broke the unity of the Muslim community and how they fought other local rulers. The rulers' subjects had little interest in joining his army; they usually felt loyal to the entire community of Islam (the umma) or else to their kin-group or village. Since the populace did not support its rulers, the rulers had to find its soldiers elsewhere, by recruiting them from outside the subject population. In short, I propose that the majority of Muslim peoples abdicated their military role. When the rulers looked outside their own domains for soldiers, they needed mechanisms for recruiting soldiers. This was the special need of Muslim rulers which military slavery filled.
Given the characteristic demand for soldiers from outside the dynasty, enslavement served as the best possible method of recruitment. In comparison with alternate methods (making alliances, paying mercenaries), enslavement had two important advantages: ease of acquisition, depth of loyalty. A ruler could more readily enslave soldiers than gain their assistance through alliance or pay. He could capture, abduct, barter, or purchase slaves; also, he could acquire them as children. The second point is vital; since children are easily molded, military slaves were made intensely loyal through training, indoctrination, conversion to Islam, and total identification with their master. Enslaving children to serve eventually as soldiers allowed the ruler to turn them into outstandingly loyal soldiers.
In brief, then, I argue that military slavery existed as a result of specific Islamic circumstances; since his subjects spurned military service, the ruler had to go out and find soldiers; for this purpose, enslavement served better than alternate methods. Because Muslim rulers alone faced this predicament, military slavery existed only in Muslim countries.
If true, this conclusion tells us something about Islamic civilization; it had a role not only in the religion and the law of Muslims, but even in their military organization. This is, I believe, the first time an Islamic element has ever been shown to play a part in military matters. It implies that for a full understanding of public affairs in Muslim countries, Islam must be taken into account.