Why were the Romans able to have a standing army, but no Medieval kingdom could?

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[White Christian Feudal states did have armies. But the Roman Army, which preceded it, was an amazing thing. The Romans could do things that Feudal states couldn't. Jan]

Because cavalry is an expensive arm. The only states which had a standing army were the Ordenstaat of the Teutonic Order (which was continuously in a war) and the Eastern Roman Empire (which was, of course, the Roman Empire).

The Roman army transitioned from a legion-based infantry army into a cavalry-centered army after the Third Century Crisis and the Barbarian incursions. Cavalry was far more efficient against Barbarians than legionaries, and the legions had only a secondary part in the battle after Diocletianus and his reforms.

Roman catafractarii, “like live sculptures of Praxiteles”.

There was one catch: Upkeeping cavalry is horribly expensive – not only the soldiers must be paid, but also the horses must be groomed, fed, equipped and trained. The Empire was in bankrupt already in the 3rd century AD, and upkeeping these live sculptures of Praxiteles to fight a defensive war where no loot, slaves and plunder was to be expected, certainly did not help it. And the logistics of a cavalry army are horrendous.

Only the Eastern Roman Empire was wealthy enough to do it. Three guesses why the ERE (Byzantine Empire) is an all-time favourite by the wargamers – and why it is wargamers and history re-enactors who know the Byzantine history best???

The Western Empire was never conquered. It simply collapsed by going bankrupt and was liquidated by its creditors – the Barbarian warlords. It had no money to upkeep even its infrastructure, not to speak about the standing army.

And when the Western Empire had collapsed, the Western Europe had been milked dry from money, noble metals and resources. It took almost 300 years to recover what the Romans had frittered away.

The most efficient way on keeping up a cavalry army is pronoia system, known as Feudalism in the West. So that every cavalryman upkeeps himself and is called to service when needed. Basically the Western Europe reverted back into the era which preceded the Marian reforms in the Roman Empire – into a conscription based military.

Conscription is a perfectly good albeit today a denigrated method to raise armies. Conscripts are not necessarily any worse in skills or morale than professional soldiers, and we all saw what happened to the professional army of Afghanistan when Taliban attacked. The armies of Israel, Switzerland and Finland are even today based on conscription.

Feudalism is essentially conscription. Every free man is a soldier. The unfree men, such as serfs, were exempt of conscription, but they could volunteer and be called to arms (arriere-ban) when situation was really dire. The feudal responsibilities meant a knight was essentially in a conscription relation to his superiors and was responsible to train and keep his fighting skills honed.

Cavalry is expensive. Feudalism was a way to outsource the costs of upkeeping the cavalry-based army to the cavalry soldiers themselves.

Infantry is cheap. Every town could upkeep town militias and require the citizens to regularly practice each weekend.

An Early Medieval army consisted of three parts. The household troops of a lord or king (which were a standing army), the men-at-arms (knights) and infantry (fyrd, ledung, heerbann, various militias etc). Of these, the household troops were the standing army, the men-at-arms were the conscripts and the infantry were the militiamen.

And, of course, there were mercenaries. World’s second oldest profession after the fille de joie. They could be hired when needed and at will.

The situation began to change in the 13th century. The Western Europe had by far exceeded the wealth and living standards of the Roman Empire, and the various kingdoms began to become wealthy enough to raise standing armies. This process was initially slow, but feudal armies were superseded finally by the early 14th century by at first mercenary and regular militia based armies, and by the mid- 15th century by standing armies.

The rise of the standing armies also meant the demise of the knight. An elite soldier who had trained all his life for martial arts, warfare, leadership skills, strategy and tactics, was a far more valuable soldier as an officer for his lessers than as a private in an elite unit. It wasn’t the firearms nor pikes nor artillery which were the death knell to knights, but becoming officers.

We – the officer corps, both active serving and reserve officers – are today the descendants of knights. And yes, many knights in the Middle Ages were mustangs, risen from the ranks and knighted on the field.

Also another answer:

Firstly, the Roman Empire had the economic muscle to back its military might. With a robust economy supported by a vast network of roads and sophisticated taxation systems, the Romans could afford to maintain a standing army. In contrast, medieval kingdoms struggled with smaller populations and less developed economies, which made it challenging for them to sustain a standing force.

Another crucial factor was the power of centralized political authority. The Romans had a strong central government that could levy taxes, raise armies, and enforces its authority. This allowed them to have a unified military under the control of the central government. In contrast, medieval kingdoms were often fragmented, with local lords and nobles holding significant power. This lack of central authority made it difficult to raise and maintain a unified standing army.

The feudal system, which was prevalent during the Middle Ages, played a role as well. Knights, who were the primary fighting force in medieval armies, owed military service to local lords in exchange for land and protection. However, knights were often not full-time soldiers and had other obligations to their lords, limiting their availability for constant military service. This contrasted with the Roman tradition of martial culture, where military service was highly valued and seen as an honorable duty for all, contributing to their ability to maintain a standing army.

Cultural and societal factors also played a part. The Romans had a strong military ethos that permeated their society, with military service being held in high esteem. In contrast, medieval societies had a different outlook, with warfare often seen as the domain of the nobility and commoners not expected to serve as full-time soldiers. This cultural difference meant that the Romans had a larger pool of potential recruits for their standing army, while medieval kingdoms relied more on a smaller class of nobles and knights for their military forces.

Technological considerations were also relevant. The Romans were known for their advanced military technology, including engineering marvels such as roads, fortifications, and siege engines, as well as superior weaponry and armor. These technological advantages gave the Romans an edge in warfare and made it easier for them to maintain a standing army. In contrast, medieval kingdoms often had more limited technological capabilities, which affected their ability to raise and sustain a standing army.

Source: https://www.quora.com/Why-were-the-Romans-able-to-have-a-standing-army-but-no-Medieval-kingdom-could