The Byzantine navy was the naval force of the Byzantine Empire. Like the state it served, it was a direct continuation from its Roman predecessor, but played a far greater role in the defence and survival of the state than its earlier iteration. While the fleets of the Roman Empire faced few great naval threats, operating as a policing force vastly inferior in power and prestige to the army, command of the sea became vital to the very existence of the Byzantine state, which several historians have called a "maritime empire".
The first threat to Roman hegemony in the Mediterranean Sea was posed by the Vandals in the 5th century. The re-establishment of a permanently maintained fleet and the introduction of the dromon galley in the same period also marks the point when the Byzantine navy began developing its own characteristic identity, a process furthered with the onset of the early Muslim conquests in the 7th century. Following the loss of the Levant and later Africa, the Mediterranean was transformed from a "Roman lake" into a battleground between the Byzantines and a series of Muslim states. The Byzantine fleets proved critical, not only for the defence of the Empire's possessions around the Mediterranean basin, but also for repelling attacks against the capital, Constantinople. Through the use of the newly invented "Greek fire", the navy's famous weapon, Constantinople was saved from several sieges and numerous naval engagements resulted in Byzantine victories.
Initially, the defence of the Byzantine coasts was borne by the great fleet of the . Progressively however it was split up into several regional fleets, while a central Imperial Fleet was maintained at Constantinople to guard the city and form the core of naval expeditions. By the late 8th century, the Byzantine navy was again the dominant maritime power in the Mediterranean. Conflicts with navies of the Muslim world continued with varying success, but in the 10th century, the Byzantines were able to regain supremacy in the Eastern Mediterranean.
During the 11th century, the navy, like the Empire itself, began to decline. The Byzantines were increasingly forced to rely on the navies of Italian city-states such as Venice and Genoa, with disastrous effects on the Empire's economy and sovereignty. A period of recovery under the Komnenos dynasty was followed by another period of decline, which culminated in the disastrous dissolution of the Empire by the Fourth Crusade in 1204. After the Empire was restored in 1261, several Palaiologan emperors attempted to revive the navy, but their efforts only had a temporary effect. Emperor Andronikos II Palaiologos even dissolved the navy completely, allowing Venice to defeat the Byzantines in two wars. By the mid-14th century, the Byzantine fleet was limited to a few dozen ships at best, and control of the Aegean Sea had passed to Italian navies and, in the 15th century, the nascent Ottoman Navy. The diminished Byzantine navy continued to be active until the fall of Constantinople to the Ottoman Empire in 1453.
Operational history Early period Civil wars and barbarian invasions: the 4th and 5th centuries
The Byzantine navy, like the Eastern Roman or Byzantine Empire itself, continued the systems of the Roman Empire. After the Battle of Actium in 31 BC, in the absence of any external threat in the Mediterranean, the Roman navy, composed of relatively small vessels, performed mostly policing and escort duties. By the early 4th century AD, the permanent Roman fleets had dwindled, so that when the fleets of the rival emperors Constantine the Great and Licinius clashed in 324 AD, they were composed to a great extent of newly built or -commandeered ships. The civil wars of the 4th and early 5th centuries did spur a revival of naval activity, with fleets mostly employed to transport armies.
Rome's mastery of the Mediterranean was challenged when Africa was overrun by the Vandals (429 to 442). The new Vandalic Kingdom of Carthage, under the capable Geiseric (), immediately launched raids against the coasts of Italy and Greece, even sacking and plundering Rome in 455. The Vandal raids continued unabated over the next two decades. The Western Empire was impotent, its navy having dwindled to almost nothing, but the eastern emperors could still call upon the resources of the eastern Mediterranean. A first Eastern expedition in 448, however, went no further than Sicily, and in 460, the Vandals attacked and destroyed a Western Roman invasion fleet at Cartagena in Spain. Finally, in 468, a huge Eastern expedition assembled under Basiliscus, reputedly numbering 1,113 ships and 100,000 men, failed disastrously. The financial cost of 130,000 pounds of gold and 700,000 pounds of silver nearly bankrupted the Empire. This forced the Romans to sign a peace treaty with Geiseric, but after Geiseric's death in 477, the Vandal threat receded.
Sixth century – Justinian restores Roman control over the Mediterranean The 6th century marked the rebirth of Roman naval power. In 508, as antagonism with the Ostrogothic Kingdom of Theodoric flared up, the Emperor Anastasius I () is reported to have sent a fleet of 100 warships to raid the coasts of Italy. In 513, the general Vitalian revolted against Anastasius. The rebels assembled a fleet of 200 ships which, despite some initial successes, were destroyed by admiral Marinus, who employed a sulphur-based incendiary substance to defeat them.
In 533, an army of 15,000 under Belisarius was transported to Africa by an invasion fleet of 92 dromons and 500 transports, beginning the Vandalic War, the first of Emperor Justinian I ()'s wars of reconquest. These were largely amphibious operations, made possible by the control of the Mediterranean waterways, and the fleet played a vital role in carrying supplies and reinforcements to the widely dispersed Byzantine expeditionary forces and garrisons. In 535, the Gothic War began with a double-pronged Byzantine offensive. Byzantine control of the sea was again of great strategic importance, allowing the smaller Byzantine army to successfully occupy the Italian peninsula by 540.
This fact was not lost on the Byzantines' enemies. Theodoric had planned to build a massive fleet directed against the Byzantines and the Vandals, but his death in 526 limited the extent to which these plans were realized. In 541 however, the new Ostrogoth king, Totila, assembled a fleet of 400 warships with which to deny the seas around Italy to the Empire. Two Byzantine fleets were destroyed near Naples in 542, and in 546, Belisarius personally commanded 200 ships against the Gothic fleet that blockaded the mouths of the Tiber, in an unsuccessful effort to relieve Rome. In 550, Totila invaded Sicily, and over the next year, his 300-ship fleet captured Sardinia and Corsica, and raided Corfu and the coast of Epirus. However, a defeat in a sea battle off Sena Gallica marked the beginning of the final Imperial ascendancy. With the final conquest of Italy and southern Spain under Justinian, the Mediterranean once again became a "Roman lake".
Despite the subsequent loss of much of Italy to the Lombards, the Byzantines maintained control of the seas around the peninsula. As the Lombards rarely ventured to sea, the Byzantines were able to retain several coastal strips of Italian territory for centuries. The only major naval action of the next 80 years occurred during the siege of Constantinople by the Sassanid Persians, Avars and Slavs in 626. During that siege, the Slavs' fleet was intercepted by the Byzantine fleet and destroyed, denying the Persian army passage across the Bosporus and eventually forcing the Avars to retreat.
Struggle against the Arabs Emergence of the Arab naval threat
During the 640s, the Muslim conquest of Syria and Egypt created a new threat to Byzantium. The Arabs conquered significant recruiting and revenue-producing areas and quickly began assembling a navy of their own. The Muslim elite largely relied on the resources and manpower of the conquered Levant (especially the Copts of Egypt), which until a few years previously had provided ships and crews for the Byzantines. Given a largely shared nautical nomenclature, and the centuries-long interaction between the two cultures, Byzantine and Arab ships shared many similarities. This similarity also extended to tactics and general fleet organization; translations of Byzantine military manuals were available to the Arab admirals.
After seizing Cyprus in 649 and raiding Rhodes, Crete and Sicily, the young Arab navy decisively defeated the Byzantines in the Battle of the Masts of 655. This catastrophic Byzantine defeat opened up the Mediterranean to the Arabs. From the reign of Muawiyah I (661–680), raids intensified, as preparations were made for a great assault on Constantinople itself. In the long first Arab siege of Constantinople, the Byzantine fleet proved instrumental to the survival of the Empire, defeating the Arab fleets with its newly-developed secret weapon: "Greek fire". The Muslim advance was halted, and an agreement to a thirty-year truce concluded soon after.
In the 680s, Justinian II () paid attention to the navy, strengthening it by resettling over 18,500 Mardaites along the southern coasts of the Empire, where they were employed as marines and rowers. Nevertheless, the Arab naval threat intensified as they gradually took control of North Africa in the 680s and 690s. The last Byzantine stronghold, Carthage, fell in 698, although a Byzantine naval expedition managed to briefly retake it. Thus, from the early 8th century on, Muslim raids unfolded unceasingly against Byzantine holdings in the Western Mediterranean, especially Sicily.
Byzantine counter-offensive
The Byzantines were unable to respond effectively to the Muslim advance in Africa because the two decades between 695 and 715 were a period of great domestic turmoil. They did react with raids of their own in the East, such as one in 709 against Egypt which captured the local admiral, but they also were aware of a coming onslaught, as Caliph al-Walid I () readied his forces for a renewed assault against Constantinople. Under Leo III (, the capital faced a second Arab siege. The use of Greek fire was again instrumental in the Byzantine victory by devastating the enemy fleet.
In the aftermath of the siege, Byzantine forces launched a counter-offensive, with a fleet sacking Laodicea and an army driving the Arabs from Asia Minor. For the next three decades, naval warfare featured constant raids from both sides, with the Byzantines launching repeated attacks against the Muslim naval bases in Syria (Laodicea), and Egypt (Damietta and Tinnis). In 727, a revolt of the provincial thematic fleets was put down by the imperial fleet through the use of Greek fire. Despite the losses this entailed, some 390 warships were reportedly sent to attack Damietta in 739, and in 746 the Byzantines decisively defeated the Alexandrian fleet at Keramaia in Cyprus, breaking the naval power of the Umayyad Caliphate. The Byzantines followed this up with the destruction of the North African flotillas and coupled their successes at sea with severe trading limitations imposed on Muslim traders. After the collapse of the Umayyad state shortly thereafter, the Byzantine navy was left as the sole organized naval force in the Mediterranean. Thus, during the latter half of the 8th century, the Byzantines enjoyed a second period of complete naval superiority.