Throughout its long history the Byzantine Empire often faced the hostility of peoples that wanted to occupy its territory. Although all emperors and senior officials preferred to settle problems with their neighbours by diplomacy, conflict was at times unavoidable. Having a well organized army and navy was considered very important.

The army
Army organization was based on two main bodies created by Diocletian: the limitanei and the comitatesii. The limitanei were farmers settled at the frontier who were given free land to farm in return for providing military service whenever  it was needed. 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 allied foreigners who served as mercenaries (they were paid for their services).
 
Emperor Heraclius changed the organization and administration of the army, introducing reforms that were completed by his successors. Newly created military units called themata were permanently stationed in the provinces to strengthen defence. The provinces were then named after the unit stationed in thema, e.g. the area where the unit from Armenia was stationed was called the Armeniac thema, the area with Thracians was named the Thracesian thema and so on. Depending on size, each thema was made up of two to four subdivisions named tourmai, led by a tourmarches (tourma commander). The tourma was divided into drouggoi, each commanded by a drungarius , and droungoi were further divided into banda. Each banda had 300-400 men and was commanded by a komes (count). At the head of each thema was the strategos (general), who was usually appointed for a period of three to four years. The troops in each thema 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 regions close to the borders 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. 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 offenses such as insubordination (disobeying orders), desertion, giving up arms etc.

Especially when on campaign, the army was accompanied by a large number of auxiliary personnel providing support services: 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 (army priests) 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 pay mercenaries. All the same, the army did offer a solution to those without money and property. 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), a writer called Kekaumenos recommends that soldiers’ pay should never be reduced, otherwise they may desert (leave their unit).

After the defeat at Manzikert (1071) the army based on themata gradually broke up and was replaced by the institution of pronoia (welfare). Under this, nobles and officials were given farms and tax money in exchange for the obligation to provide military service in wartime. This measure was unsuccessful – the army shrank to the point where it relied mainly  on mercenaries who only fought for money and, not because they wanted to save their country.

The navy
The spread of the Arabs in the 7th century threatened Byzantine control over the sea and led to the reorganization of the ploimon (navy), headed by the Strategos karabisianon (General of the ships). The navy was organized as an integral part of the thema 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 could carry up to 300 men, warriors and rowers. Other types of warships were the dromonion and chelandion.

Every time the fleet was about to leave, commanders would inspect their ships and men, while priests chanted a particular psalm to ensure their success and safety. 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.