List of sieges of Constantinople in the context of Free city (classical antiquity)

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, but their threat was ended by the wars of Justinian I in the 6th 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 departing from its late Roman roots and developing its own characteristic identity. This process would be 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. In this struggle, the Byzantine fleets were critical, not only for the defence of the Empire's far-flung possessions around the Mediterranean basin, but also for repelling seaborne attacks against the imperial capital of Constantinople itself. Through the use of the newly invented "Greek fire", the Byzantine navy's best-known and feared secret weapon, Constantinople was saved from several sieges and numerous naval engagements resulted in Byzantine victories.