For the better part of a millennium the
Except that a strange thing happened as the equites grew more affluent. In time, they grew less willing to send their own sons and horses to fight, and more willing to stay at home and pay their taxes so that plebians might be hired in their place and given government-owned horses. In time, the Empire grew so large and far-flung and the needs for manpower grew so vast that it became far more cost-effective to arm and train the plebians—and increasingly, foreigners—to fight on foot, and to reduce the cavalry to a fast and elite striking force. In time, the equestrians began to see themselves as the true nobility, and to build larger and better-walled estates even further out in the provinces, for which they hired their own guards and retainers who were provided with privately owned horses and increasingly, privately owned weapons.
In time, the Empire fell apart, and Latin disintegrated into local dialects, and the horses became cavallos in Italy, caballos in Spain, and chevals in France, and the men who rode and fought on them became caballeros, or chevaliers, or cavalry depending on where they lived, and what would in time be called the Age of Chivalry was born, although for most of the people who lived under feudalism life was nasty, filthy, brutal and short.
Why all this emphasis on horses?
Because an armed and trained man on a well-trained horse is a formidable weapons system, and a deadly menace to men on foot. A group of men on horseback, trained to act as a unit, can overwhelm a force of infantry many times their size, especially in an age when "foot soldiers" and "disorganized rabble of peasants" were interchangeable terms. Never mind the man on its back; the sight of a thousand-pound animal charging down on you is terrifying, especially if all you have to protect yourself is a pointed stick.
For some centuries a leisurely sort of arms race was carried on between cavalry and infantry, although it must have seemed terribly quick to the men directly involved. The pointed sticks became halberds and poleaxes; the cavalry upgraded their armor and went to longer lances and heavier swords. The infantry began to carry crossbows; the cavalry upgraded their armor again and began to armor the heads and breasts of their horses. The increased armor was heavier, so the breeders began producing larger horses, Percherons and the like, to carry the extra weight—but a larger horse also eats more, and so the cost of putting an armored fighting man on horseback in the field continued to escalate, even as the weapons, tactics, and training of the men on foot improved.
In the end, as always, it was a technological revolution that settled the question. The technology in question was the English longbow, which was the rapid-firing assault weapon of its day, and the Age of Chivalry ended decisively in the 14th Century with thousands of over-armored French knights, unhorsed and helpless as turtles on their backs, dead in the mud of Crécy, Poitiers, and Agincourt. Yet even as they died, the next technological revolution was already waiting in the wings, as Crécy was the first known battle fought to the sound of primitive English cannons booming in the background, sometimes effectively.
Still, it would take another five centuries, and the hideous spectacle of the British Light Brigade charging dug-in Russian guns in the Crimean War and the French and Polish cavalry charging dug-in German machine guns in the First and Second World Wars, before the western world's military forces finally, collectively, gave up on the 2,000-year-old idea of the noble equestrians, and war horses became polo ponies and pets for rich children.