The Median army was the primary and foundational model upon which the Achaemenids built their own military structure. This army was in itself a formidable force, responsible for the destruction of the powerful Assyrian Empire and paving the way for the re-emergence of an Iranian power on the world stage. In its initial stages, the military structure of the Medes was based on a loose coalition of tribes. In this structure, military organization was shaped more by clan and tribal affiliations than by specialization and weapon type. Every free man who could bear arms was considered a warrior.
However, the constant threat and devastating attacks of the highly organized Assyrian war machine acted as a catalyst for the unification and professionalization of the Median military. The need for a coordinated defense forced the scattered Median tribes to gather under a single leadership. Deioces, the founder of the Median Kingdom, understanding this necessity, transformed Ecbatana (modern-day Hamadan) into a strong military fortress and administrative center to provide a centralized response to the Assyrian threats.
The turning point in the military history of the Medes was the fundamental reforms of Cyaxares. He transformed the Median army from a tribe-based guerilla force into a regular, professional army. This transformation is considered a key moment in the military history of Iran. Cyaxares, understanding the importance of specialization, organized the army into separate units of spearmen, archers, and cavalry. This organization allowed the Medes to employ more complex tactics beyond simple mass charges.
Furthermore, he incorporated advanced siege techniques into his army, which he had likely learned from the Assyrians themselves. This new capability gave the Medes the power to conquer fortified cities like Nineveh, the capital of Assyria, in 612 BCE, and to end Assyrian domination forever. The Median army was known for several key features that made it a formidable force in its time.
The superiority of the Median cavalry had reached world fame. They were the first breeders of the famous Nisean horses; horses so powerful that they were a source of terror even for the Assyrians. The Median cavalry likely served as both a shock force and as skilled horse archers. The main weapons of the Median soldiers included spears, bows and arrows, short swords (Akinakes), and javelins. Their defensive equipment included large shields (likely made of wood or leather) and, for the elites, a type of armor, perhaps made of leather or linen.
Based on later descriptions and reliefs, a Median soldier typically wore a tunic down to the knees, tight trousers, and a soft, pointed, round cap on his head, the flaps of which could cover the ears and chin. The evolution of the Median army cannot be analyzed without considering two key factors. First, what can be called the "effect of the Assyrian furnace." The development of the Median army was a direct and evolutionary response to the existential threat of Assyria.
Second, the Median army under the leadership of Cyaxares was something more than a national army; it was a "pre-imperial force" that integrated allied and tributary forces such as the Scythians and, most likely, the Persians. Cyaxares first defeated the Scythians and then incorporated a part of them into his army. The alliance with the Babylonians was crucial for defeating Assyria, and it is very likely that the Persians, as vassals of the Medes, provided military services in these campaigns.
This evidence shows that the Medes, before the Achaemenids, had developed the key imperial skill of integrating diverse military forces for a common goal; a practice that the Achaemenids would later perfect on a global scale.