Byzantium was continuously at war, and without its army it would not have survived. The key to the army’s success was its ability to change, as the face of the enemy changed. The fourth century saw the first change, in response to the greater mobility of
The army, composed of indigenous and federate troops (foederati, recruited from barbarian allies) used frontier militias (limitanei) to protect the borders, while mobile troops (comitatenses) commanded by the emperor, with his elite imperial bodyguard, gave chase to intruders. At least this is how it was meant to work in theory. In practice the response of mobile field forces was not always quick enough, and some units became stationary residents of areas for long periods of time. Moreover, emperors of the fourth and fifth centuries increasingly left campaigning to a German magister militum, some of whom, like Aspar, proved to be unreliable, even treacherous. In any case, even greater mobility was required to deal with the Persians and Avars, each of whom relied on cavalry and archery. This prompted the organization (beginning in the seventh century) of vast military districts called themes, whose armies were meant to provide a defense-in-depth against invaders. The soldier-farmers of thematic armies were supported by the revenues supplied by small estates (stratiotika ktimata, literally “soldiers’ properties”), given in return for hereditary military service. Thematic armies were slow to mobilize and their commanders (strategoi) were so often involved in revolts in the eighth century that their effectiveness is questionable. In any case, against Arab raiders more mobility was needed, hence the development of mobile field forces (tagmata) stationed around Constantinople. Cavalry was an important part of the tagmata. Their role was to follow the Arab raiders, dogging their every move, delaying them until the combined tagmata and thematic troops could strike. Nikephoros II Phokas developed a form of heavy cavalry called kataphraktoi, covered with mail or lamellar coats, and with iron helmets; their horses were protected as well. They operated in flying wedges, followed by the regular cavalry, which could disintegrate an opponent’s ranks. If this sounds well thought out, it was. Strategy and tactics in Byzantium were the object of scientific study, as indicated by the military manuals (strategika) that have survived. From the 10th century down to the end of Basil II’s reign in 1025, the army, although much reduced in numbers from fourth-century totals, was superior to any in the western world.
The decline of the army began before 1025 with the growth of large estates in Asia Minor at the expense of soldier-farmers who were the backbone of the thematic armies. The military aristocracy (the dynatoi) were among the chief offenders, and they included the likes of Bardas Skleros and Bardas Phokas, whose revolts almost unseated Basil II. The struggle between the central government and the military magnates intensified after Basil II’s death, continuing until 1081, just as new threats were appearing. Basil II’s successors, including Alexios I Komnenos, were forced to rely chiefly on massive numbers of mercenaries, as the old tagmata and thematic armies shrunk to insignificance. One exception to this were the “Macedonians,” indigenous troops from the western provinces, levied largely in Thrace. Beginning with Alexios I, the importance of the Varangian Guard increased, as numerous western soldiers-of-fortune appeared, like Hervé Frankopoulos and Roussel de Bailleul. In addition to westerners there were soldiers from the steppe, e.g., Pechenegs, Cumans, and Uzes, many of whom may have been hired for individual campaigns rather than maintained continuously in the army. From the Balkan Peninsula other mercenaries were recruited, e.g., Serbs, Hungarians, Alans, and Vlachs. Alexios I created from this ethnic mix an effective army that campaigned for over 30 years. John and Manuel Komnenos inherited this new army, and they combined it with an aggressive program of fortification in areas endangered by enemy attacks. Field armies were then used chiefly to besiege fortified places in enemy hands, and garrison them once they were conquered. As Byzantium was further weakened by the Crusades, its ability to purchase mercenaries declined. Use of pronoia grants increased accordingly. By the late 14th century the Ottomans were fielding huge armies, much larger than Byzantine forces, pitifully small by comparison, could resist.
(Source: «Historical Dictionary of Byzantium», by John H. Rosser)