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.
July 22, 2021 update: For the first time since I published Slave Soldiers and Islam in 1981, I find another scholar has re-asked the questions at the base of my study: why did military slavery exist and why was it distinct to Muslim rulers?
That would be Feifei Xu, a historian at Sichuan University in Chengdu, China, in a nine-page paper, "On the Uniqueness of Military Slavery of Medieval Islamic World," presented this year at the 4th International Workshop on Advances in Social Sciences (IWASS).
One of the main reasons for the great success of Islam in its early days was the appeal to its followers of the vision of Umma, a society of political integrity and equality. ... However, since Muʻāwiya seized power by force and established the hereditary Umayyad Caliphate, the rulers were no longer undisputed religious and political leaders of Umma, but monarchs who ruled by force. Civil wars followed, and the Abbasids who replaced the Umayyads were no more popular than the latter, thus the ideal community of unity was gone. ...
With the gradual separation of politics and religion, the functions of the caliphs as religious leaders were gradually transferred to the ʻUlamā' class. The caliphs were more political leaders, and the later sultans were purely secular rulers. The disillusionment of the ideal of Umma has left the Caliphate government unpopular and short of cohesion. Up to now, religion still holds the greatest appeal among the masses of Muslim countries, while secular ideas can only be easily accepted by a few elites. In order to maintain the rule, the rulers sought new sources of support, and the professional military group became the pillar of royal power. ...
After the decline of the Abbasids, the sultans and emirs in Muslim world were all secular rulers, who also faced the problem of lack of cohesion caused by the separation of religion and state, so they all adopted military slavery to maintain their rule. In Muslim countries, juntas are always proponents of secularism and a counterweight to religious force, but in the form of military slaves in pre-modern era. The essence of military slavery is a kind of secular political force used by rulers to balance the religious power in pre-modern Muslim society.
In brief, Xu agrees with my analysis.
Nov. 26, 2022 update: In an undated paper, "Separation of Powers and the Medieval Roots of Institutional Divergence between Europe and the Islamic Middle East," Eric Chaney argues that "rulers in the Islamic world developed a system of slave armies to avoid a European-like separation of powers."
