This book by Max Boot, just out, is on my Summer list. In the mean time Boot wrote an article on the subject for Foreign Affairs:
The success of various raiders in attacking and conquering states from ancient Rome to medieval China gave rise to what one historian has called “the nomad paradox.” “In the history of warfare, it has generally been the case that military superiority lies with the wealthiest states and those with the most developed administrations,” the historian Hugh Kennedy wrote in Mongols, Huns, and Vikings. Yet going back to the days of Mesopotamia, nomads often managed to bring down far richer and more advanced empires. Kennedy explains this seeming contradiction by citing all the military advantages nomads enjoyed: they were more mobile, every adult male was a warrior, and their leaders were selected primarily for their war-making prowess. By contrast, he notes, settled societies appointed commanders based on political considerations and drafted as soldiers farmers with scant martial skills.
Nomads’ military advantages seem to have persisted among guerrillas in the modern world; even in the last two centuries, during which states became far more powerful than in the ancient or the medieval period, guerrillas often managed to humble them. Think of the tribes of Afghanistan, which frustrated the United Kingdom, the Soviet Union, and the United States. Kennedy’s “nomad paradox” is really a guerrilla paradox, and it asks how and why the weak seem to so frequently defeat the strong. The answer lies largely in the use of hit-and-run tactics, taking advantage of mobility and surprise to make it difficult for the stronger state to bring its full weight to bear.
Guerrillas often present a further paradox: even the most successful raiders have been prone to switch to conventional tactics once they achieve great military success. The Mongols eventually turned into a semiregular army under Genghis Khan, and the Arabs underwent a similar transformation. They fought in traditional Bedouin style while spreading Islam across the Middle East in the century after Muhammad’s death, in 632. But their conquests led to the creation of the Umayyad and Abbassid caliphates, two of the greatest states of the medieval world, which were defended by conventional forces. The Turkish empire, too, arose out of the raiding culture of the steppes but built a formidable conventional army, complete with highly disciplined slave-soldiers, the janissaries. The new Ottoman army conquered Constantinople in a famous siege in 1453 and, within less than a century, advanced to the gates of Vienna.
Why did nomads so adept at guerrilla tactics resort to conventional warfare? For one thing, their targets became bigger, requiring a shift in tactics. Mounted archers could not have taken Constantinople; that feat required the mechanics of a proper military, including a battery of 69 cannons, two of which were 27 feet long and fired stone balls that weighed more than half a ton. Nor were fast-moving tribal fighters of much use in defending, administering, and policing newly conquered states. Those tasks, too, required a professional standing army. A further factor dictated the transformation of nomads into regulars: the style of fighting practiced by mounted archers was so difficult and demanding that it required constant practice from childhood on for an archer to maintain proficiency. Once nomads began living among more sedentary people, they “easily lost their superior individual talents and unit cohesion,” write the historians Mesut Uyar and Edward Erickson in A Military History of the Ottomans. This was a tradeoff that most of them were happy to make. A settled life was much easier — and safer.
The nomads’ achievements, although great, were mostly fleeting: with the exception of the Arabs, the Turks, the Moguls, and the Manchu, who blended into settled societies, nomads could not build lasting institutions. Nomadic empires generally crumbled after a generation or two. Former nomads who settled down found themselves, somewhat ironically, beset by fresh waves of nomads and other guerrillas. Such was the fate of the Manchu, who, as the rulers of China, fought off the Dzungar (or western Mongols) in the eighteenth century and tried to fight off the Taiping rebels in the deadliest war of the nineteenth century. The Taipings, in turn, tried to develop more powerful armies of their own, blurring the distinction between regular and irregular conflict. Since then, many civil wars, including the one the United States fought between 1861 and 1865, have featured both kinds of combat.
Much more at the link if you’re at all interested in the topic.