Could medieval soldiers be organized into a Roman style legion order of battle?

Question

Is there any reason why medieval soldiers could not have been organized into the order of battle of the Roman Empire by a ruler or empire that chose to do so? Integrating crossbowmen, longbowmen, spearmen, light cavalry, heavy cavalry (Knights), etc.

I know the Komnenian era had a relatively organized army in the Eastern Roman Empire, but they were not at the scale of the professional standing armies of the Roman Empire from my understanding nor were they organized into an order of battle as organized as that of the Roman legions. Perhaps an expert on the subject matter could explain if the Komenian era Eastern Roman Army was as organized as the Roman legions or not.

Is there a reason that the Legion order of battle model could not be applied to medieval warfare though adjusted for more cavalary? The tactics and strategy would of course change. The table of organization would also likely change from the structure of the Roman Legion as a medieval Legion would integrate more heavy cavalry, crossbowmen, longbowmen, etc.

Answer 1

NO.

The Roman army was made up of "professional" soldiers, who served 25 years (from their late teens to their early 40s, like modern ball players), before they were disbanded. No medieval armies had soldiers of this standing, although the Kommenians came closer than others.

This started after the Punic Wars, when cheap grain acquired from Sicily (and other points south of Rome) "dispossessed" the sturdy yeoman farmers. They had to find other pursuits, with the military being one of the more obvious ones, hence the Roman practice of "career" soldiers.

The feudal societies of medieval Europe had no "Sicilies" to produce surplus grain, meaning that essentially all of their men were needed for farming, and only a handful of nobles could afford to be "professional" soldiers. Imagine an army where only the officers were professional, and essentially all of the recruits (95% of the army) had less than a year of training and experience, and were basically recruited for a single battle. That was the difference between Roman and medieval armies.

Answer 2

Long time ago I wondered about the same. I always thought the Roman army (early imperial, of course) would beat the crap out of any opponent until the end of the middle ages. After learning a lot about history I had to change my opinion completely.

No. 1: The Roman army was entirely professional. From the lowest recruit up to generals in command. Medieval armies were definitely not. That was a professional cadre (nobles, knights and possibly sergeants) leading drafted poorly equipped and poorly trained peasants. And poorly led, certainly in the eyes of Roman professionals.

No. 2: Logistics. That concept went completely down the drain after the fall of Rome in the West. (Do mind that the Byzantine army was a continuation of the Roman army with excellent logistics and survived a millennium). For a professional army you need a lot more than pack a lunch.

It's there that medieval armies fell short. It was something they simply couldn't do, no matter what. You need supplies for the army in the field. Transportation to get it there. Folks back home producing food, arms, uniforms and equipment. I leave out medical services, medieval armies weren't very strong in that department. No medieval state except for Byzantium was capable of doing that.

It took about a year to train a Roman recruit to be a basic legionary. During that year they did nothing else but train. They had to eat now and then, meaning that someone else had to grow the food, harvest it and get it to the legion. Once trained and professional, they had to keep training to remain fighting fit. You can't be a farmer and a professional legionary. It's either - or. Medieval states had no capability for that. Rome and Byzantium did.

Another very important point here: if you hire soldiers full time, until they have to retire due to (relatively) old age at 40-50, you take their best and most profitable years. That means you have to provide them with some sort of pension. That was within medieval society completely impossible.

3- (Very) early medieval armies actually were (very) late Roman armies.

The Roman army varied over time. It adapted itself to the enemies it had to face. From a hoplite army suitable to fight local wars into a more cavalry based relatively mobile army at the end.

During the battle of Chalons against the Huns the Roman army was present but didn't do much. Even Aelius didn't think they were of much use other than be an obstacle. A rather inglorious end for the probably best army ever...