When the Roman Empire collapsed in AD 476, Europe was shattered into thousands of tiny kingdoms. Over time, these tiny kingdoms grew, either by merging through marriage or by conquest. And thus, by the 10th century, the feudal system was in place all over the continent.
Under this system, a lord (or king) had absolute control over a span of territory that ranged in size from a few square miles to several hundred square miles. Underneath the lord were the clergy and knights, who protected the lord both spiritually and physically. Underneath them came the merchants. Underneath the merchants were skilled laborers, like blacksmiths and coopers. And underneath the skilled laborers were the unskilled laborers, or serfs, who farmed the lord’s land.
It might initially seem that the feudal system was a one-way hierarchy. After all, the serfs could not move (or even travel, in most cases) without their lord’s permission. They grew crops on the lord’s land, and were required to give him a large portion of their harvest to pay rent. But the feudal system had a paternal side, too. The lord was expected to treat the serfs fairly, and to use his knights to protect them from invading armies or bandits.
In time, these small societies grew in size, as one small kingdom was conquered and absorbed into another, or as marriage created associations between such kingdoms. Eventually, these small kingdoms would grow into the modern countries of England and France (and eventually Germany and Italy, although those two countries wouldn’t be fully formed until the 19th century).
Here’s the interesting thing, though: from around AD 900 until the American Civil War, most armies were based on that feudal system. Instead of a “national army” made up of “English” soldiers, most armies were made up of thousands of individual regiments formed by dukes, earls, and other forms of nobility. As soon as a king knew that an invasion was coming, he’d put out a call to his aristocracy, who would organize their own knights and take them wherever the king wanted them. Although the soldiers were certainly loyal to their king, they generally only took orders from their local lord.
Although this system would break down over time (especially by the time of Napoleon, the founder of the first “modern army”), traces of it nevertheless persisted until the US Civil War, when Irish immigrants were allowed to create their own regiments for the Union army. So instead of “Lord Fluffernutter’s Essex Cavalry” you had “O’Connor’s 96th Irish Infantry”. So… slightly different, yet the same.
Here’s what’s really interesting, though. There is still one such “feudal army” in Europe: the Atholl Highlanders.