Adam Smith's "Wealth of Nations" is not only a great founding text of economics, but a deep well of knowledge about how society functions in its manifold ways. His historical analysis of warfare is a case point.
Traditionally, civilization is divided into three stages: hunter-gatherer, agricultural, and industrial. Smith, however, argues that there's an important intermediate stage that is often overlooked, which has exerted an enormous influence of world history, and deserves to be recognized in its own right. This is the stage of civilization which he calls the nations of shepherds, or the sheepherding society.
Sheepherding societies are nomadic societies that tend huge flocks of animals, whether it be sheep, horses, or cattle. They wander from region to region, letting their herds exhaust a grass-field before moving onto the next. Unlike hunter/gatherer societies, sheepherding societies are masters of their own fate, their survival depending on their ability to maintain their herds.
In terms of pre-industrial warfare, sheep-herding armies are the most lethal. Unlike hunter/gatherer societies, sheepherding societies have the capacity to form war-making juggernauts that easily over-run agricultural civilizations surrounding them. Most members of a sheep-herding society are used to outdoor mobilization. They can handle weapons and the society offers little opportunity but to herd sheep or fight wars. But most importantly, as logistics is the heart of warfare, sheepherding armies have a decided mobilization advantage as they do not have to worry about maintaining a supply-line because their food source – sheep – carry themselves along on the journey .
Smith argues that sheepherding societies are also economically incentivized to form even more strongly hierarchal societies than agricultural societies. The reason is that the limited diversity in production for sheepherding societies means the only outlet for a sheepherding king to accrue wealth can only be expressed in terms of more sheep or more soldiers. There are no pretty trinkets, golden treasures and magnificent buildings that he can spend his wealth on. As such, the economic incentive for a successful king to show his wealth is to increase the size of his herd and soldiers under his tutelage – the perfect ingredients for building an army. Although sheepherding societies are naturally primed for warfare, most of the time, they fritter away their strength in an unending cycle of internal wars.
Smith argues that many of the great empires in history have originated from sheep-herding societies – the Aryans that conquered Assyria, the Mongols, and the early Islamic empire. These are examples where a particular powerful and charismatic leader manages to unite a large group of sheep-herding tribes, thus forging a mobile battle-ram through the agricultural regions around them. It is no accident that these are all sheepherding societies that have embraced the horse. Allegedly the Aryans brought the chariot, whilst the Mongols were masters of the horse. Whilst history books may seem surprised by the humble sheep-herding origins of these empires, Smith argues that it is almost inevitable given the socio-economic advantages of building empires on the back of a sheep.