As this particular topos goes, originally war was waged by men of poor, nomadic tribal societies like those of which, long ago, all of us used to be a part. At first they fought over such things as access to water, hunting- and grazing ground, domestic animals, and, not least, women. At some stage one tribe, often headed by a particularly able leader, defeated all the rest and united them into some kind of league, confederation, or federation. As the ancient Assyrians, Babylonians, Persians, Huns, Magyars, and Mongols all did.
Next, the victors took on their richer, settled, neighbors. They fought, triumphed, conquered, and subjugated. Having done so, they discarded their nomadic traditions and took up life in the cities under their rule. Exploiting the labor of others, they grew rich and soft. They also indulged in every kind of luxury, allowed themselves to be governed by women, and witnessed a sharp decline in fertility.
Having abandoned the military virtues, at some point they started looking down on them. Hiring foreigners to fight in their stead, they ended by losing the qualities that had made them great. Attempts to substitute technology for fighting power, such as were made both in fourth-century AD Rome and, repeatedly, in China, did not work. Nor is there any reason why they should, given that the barbarians could often capture or imitate the technologies and find renegades to operate them. As, for example, Genghis Khan and Timur did. Each empire in turn was overrun by its poorer, but more virile and aggressive, neighbors. More often than not subject peoples, long oppressed, rose and joined the invaders. The end was always the same: ignominious collapse.
Source: Topoi – Martin van Creveld
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