Throughout its long history the Byzantine Empire often faced the hostility of peoples that wanted to occupy its territory. Although the emperor and senior officials relied mainly on diplomacy to deal with the hostile inclinations of their neighbours, there was a pressing need for a well organized army and navy.

The army
After the reforms of the Tetrarchy, military organization was based on two main bodies created by Diocletian : the limitanei and the comitatesii. The limitanei were farmers settled at the frontier, to whom the state made grants of arable land in return for military service. The comitatesii were the regular army under the emperor’s command, which was sent wherever necessary. Other army units called the bucelarii and the foederati were made up of foreign mercenaries . The organization and administration of the army changed in the reign of Emperor Heraclius . The themes, aimed at readying and reinforcing defence, were military units stationed in the provinces. The provinces were then named after the unit stationed in them, e.g. the area where the unit from Armenia was stationed was called the Armeniac theme, the area with Thracians was named the Thracesian theme and so on. Depending on size, each theme was made up of two to four subdivisions named tourmai, led by a tourmarches (tourma commander). The tourma was divided into droungoi, each commanded by a drungarius, and droungoi were further divided into banda. Each banda had 300-400 men under the orders of a komes (count). The head of military and political administration in each theme was the strategos (general), who was usually appointed for a period of three to four years. The troops in each theme were made up of professional soldiers stationed at the general’s headquarters and at strategically important city-castles. There were also farmer soldiers, settled on lands granted by the state in return for providing military services in wartime. In border regions there were military corps named kleisoura (a Greek word also meaning a narrow pass between mountains), commanded by a kleisourarchis. Closely related to them were the Akritai , military units of border guards.

From the 9th century onwards the main imperial forces were the battalions, in other words military units in Constantinople and other major imperial cities consisting of elite units. The most important of these were: a) the Scholae, a body of both cavalrymen and foot soldiers; b) the Excubitores (“Sentinels”), usually sent on confidential missions; c) the Number or Vigla Corps, whose main task was to guard the palace; and d) the Hikanatoi (“Able ones”), which was the newest guard corps. Domestikoi and droungarioi were the commanders of these bodies. The emperor’s personal guard was made up of foreign guards called the Hetairia (“Companions”). The tasks assigned to the various military corps were very well defined and command was based on order and discipline, with penalties for offences such as insubordination, desertion , giving up arms etc.

Especially when on campaign, the army was accompanied by a large number of people providing support services and logistics: the so-called touldon, made up of farriers, blacksmiths, cooks, nurses, and animals carrying equipment (tools, tents, food, weapons, etc.). Other members of the support staff were cantatores (orators and preachers) to animate the soldiers; mandatores (messengers) who saw to communication between corps; artists to entertain the warriors; and chaplains who worked in the camps and on the battlefield. Soldiers were also allowed to take slaves or servants with them to serve various practical needs.

Military service lasted several years. In order to avoid it some people became monks or even had limbs amputated, while the rich bought their way out of military service with money that the state used to support and pay for volunteers. However, the army service offered a way out for the less fortunate, especially during difficult financial times. Apart from their regular salary, the roga, which depended on rank and corps, soldiers received part of the spoils, tax exemptions and rations (bread, wine, olive oil and meat). The military also received extra benefits from the emperor when he was enthroned or got married, or when special events were celebrated. In Logos Nouthetetikos (a speech giving advice to an emperor), Kekaumenos recommends that soldiers’ pay should never be reduced, otherwise they may defect .

After the defeat at Manzikert (1071) the army based on themes gradually broke up and was replaced by the institution of pronoia (welfare). Under this, nobles and officials were given farms and tax revenue in exchange for the obligation to provide military service in wartime. Combined with the fact that the army increasingly relied on mercenaries, the failure of this means of conscription led to the army contracting and gradually losing its combat effectiveness.

The navy
The spread of the Arabs in the 7th century threatened Byzantine control over the sea and led to the reorganization of the navy, headed by the Strategos karabisianon (General of the ships). The navy was organized as an integral part of the theme administration system. This created fleets organized by province - the thematika ploima - under the command of generals or droungarioi. The imperial ploimon was an independent fleet used for defending the capital city; it was led by the drungarion tou ploimon, or Admiral General. The main warship during the Byzantine period was the dromon (runner), which had two or three sails and two banks of oars. Depending on size it could carry up to 300 men, warriors and rowers. At the base of the main mast dromons had wooden towers, the xylokastra, from which soldiers could hurl spears and arrows at enemy ships. They were also equipped with containers of Greek fire, which did not extinguish on coming into contact with water and was highly effective when there was no wind. Other types of warships were the chelandion, bigger and heavier than the dromon, mainly used for landings, and the pamphylos, a speedy, well equipped ship, which had elite crews and skilled warriors.

Before the fleet departed, commanders would inspect their ships and men, while priests would say prayers to ensure their success and safety and offer reliquaries with splinters of the true cross and saints’ relics for the men to do obeisance. The most common way of giving naval battle was called pelagolimin: the war fleet was arranged in a semicircle, with the admiral’s dromon placed in the middle so that the orders he gave were heard by all. The strongest dromons were positioned at both ends of the formation. Another formation was the side-on attack, with the bows facing the enemy. In contrast to ancient times, naval tactics no longer relied on ramming , but on coming up alongside enemy ships and tying up to them. When this had been done, soldiers would rush on board and give battle man-to-man.

The Byzantine Empire’s naval power decreased over time. By the end of the 13th century the navy had been broken up, since the state could no longer afford to keep it up. This allowed the Venetians and the Genoese to control the Mediterranean, even though in the past they had sometimes fought on the same side as the Byzantine fleet.