At the end of the civil wars, Octavian had nearly 700 warships of all types, far more than would ever be needed; the numbers were reduced by laying up, scrapping or simply burning surplus ships. Octavian adopted the title Augustus in 27 BC and with Agrippa reorganised naval forces to form the imperial fleets. There followed a period, unequalled before or since, of approximately three centuries during which that navy was to exercise unchallenged supremacy over the whole Mediterranean basin, extending its influence in time to encompass the Black, Red, North and Irish Seas, the English Channel and north-west Atlantic seaboards, as well as the great river frontiers of the Empire, the Rhine and Danube (Danubius). These forces reigned supreme until in the late second century AD, when the first barbarian raiders from across the North Sea were noted, but they were not then enough to provide any serious challenge.

The fleets, apart from policing the seas, were also engaged in offensive operations, starting in 25 BC, with the transfer of a fleet of eighty warships to the Red Sea to support 130 transports and a military expedition to Sabaea (probably modern Yemen). In 17 BC Agrippa commanded a fleet which landed marines in the Crimea to resolve a dynastic problem in the Bosporan kingdom, a client state. Between 17 and 15 BC Augustus advanced the Roman border to the line of the Danube, forming river fleets for it. Fleets operated along the Dutch and German coasts as well as into the rivers of Germany after 12 BC in support of Augustus’ short-lived province there. With peace and security in the Mediterranean came an increase in maritime trade, reaching in the first two centuries AD, a level not surpassed until the nineteenth century. Merchant ships grew in size as well as in numbers and regular sailing schedules for cargo and passengers became the norm.

The next major challenge for the navy was in AD 43 with Claudius’ invasion of Britain; for the invasion army of four legions plus auxiliaries (about 40,000 men in total) a fleet of over 300 warships and transports was assembled. Apart from ferrying the army, it would be the sole means of supply, the invasion forces not planning to live off the land at all. The fleet was also to accompany the army’s advance along the south coast and up the Thames (Tamesis) guarding the flanks, outflanking any opposition and supplying the troops. These formations, which would later become the Classis Britannica, continued the process of keeping the flanks of the Roman advance secure throughout the invasion, with fleets on both east and west coasts of Britain.

The navy supported Claudius’ annexations of Mauretania (the coastal regions of Algeria and Morocco) in AD 41 and 42, Lycia (southwest Turkey) in AD 43 and Thrace in AD 46, giving Rome control of the whole Mediterranean seaboard. The extension of Roman naval power in the Black Sea was completed by the founding of a major naval base at Chersonesus (near Sevastopol, Crimea) in AD 45 and the annexation of the kingdom of Pontus (on the north Turkish coast) in AD 64. By this time also, Roman warships were operating in the North and Irish Seas. As a portent of things to come, however, in AD 41 tribesmen from the north German coast, using small open boats, raided the coast of Belgium before being driven off. They repeated the exercise in AD 47, burning an auxiliary fort at the mouth of the Rhine but were caught by ships of the Classis Germanica and destroyed.

In AD 59 Nero (emperor AD 54–68) conspired with his prefect of the Misene fleet, Anicetus, to murder his domineering mother, Agrippina. The first attempt using a collapsible boat failed when she proved to be a strong swimmer. Anicetus then had his sailors simply batter her to death. Anticipating unrest to come, Nero formed two legions, I and II Adiutrix, from loyal marines of the Misene and Ravenna fleets respectively, but committed suicide in AD 68. This triggered the events of the so-called Year of the Four Emperors, which involved the Italian and Rhine fleets. Nero’s ‘navy’ legions surrendered to his successor Galba but suffered casualties at the hands of his men. By January 69 Galba was dead, succeeded by Otho, who regained the loyalty of the Misene fleet and naval legions. He moved north to oppose the advance of Vitellius, proclaimed emperor by the Rhine fleet and legions. Otho sent the Misene fleet to secure southern Gaul for him but with poor morale and lax discipline, the fleet took to raiding and plunder. After two fierce battles against Vitellius’ forces, the fleet withdrew. Otho, relying heavily on naval personnel, was defeated at the battle of Cremona in April AD 69, leaving Vitellius emperor.

In the east, in July AD 69, Vespasian was proclaimed emperor and an army supporting his claim, proceeded to Italy, supported by the Classis Pontica, gathering further support as it advanced. Vitally the Ravenna fleet declared for Vespasian, securing his forces’ flank and lines of supply and tipping the balance firmly in his favour. Vitellius was defeated at the second battle of Cremona in October AD 69, but with his remaining supporters, continued the struggle until killed that December. For the Misene fleet it was a time of confused loyalties, following their ignominious support of Otho. Factions pulled the fleet between Vitellius and Vespasian until the latter’s cause prevailed.

In the meantime, there arose a serious revolt led by Iulius Civilis, a commander of Batavian auxiliary troops (from north-eastern Belgium), who seized some of the German fleet’s ships and occupied the mouth of the Rhine. With the ships, augmented by the boats of German tribesmen from east of the Rhine, he pushed upriver against garrisons weakened by troops having left to support Vitellius. Defections by other auxiliary crews and troops, forced the remains of the fleet and legionaries to withdraw to Bonn (Bonna) where they were beaten by the rebels, again joined by Germans from east of the river. Xanten held out and although the fleet sallied successfully against the rebels, it had insufficient strength to be decisive. The Romans fell back on Trier (Augusta Treverorum) with their fleet at Koblenz (Confluentes).

With the end of the civil war, forces could be sent to restore order: the Classis Britannica landed troops in Belgium, pushing the rebels back but in the process, losing a lot of their ships to an attack by tribesmen from what is now Friesland. The Rhine fleet remained in poor condition and lost some moored ships to German raiders. With increasing Roman success, it recovered and sailed against Civilis’ fleet. Although coming close to each other and exchanging missiles, conditions did not permit an engagement. Civilis surrendered shortly afterwards.

The navy resumed its normal ‘peacetime’ duties and operations. The Classis Pontica secured the south-east Black Sea area and in AD 79 the Misene fleet assisted in rescue attempts during the great eruption of Vesuvius that buried Pompeii and the surrounding area. Ships of the Classis Britannica circumnavigated Britain in AD 80 and reconnoitred Scotland and Ireland two years later. The Classis Germanica was in action repelling Germanic incursions across the Rhine and the Classis Moesica had helped in stopping a Dacian invasion across the Danube in the late eighties AD.

The beginning of the second century AD was marked by the Dacian Wars (AD 101–106) of Trajan (emperor AD 98–117). The Danube fleets were augmented by ships seconded from other, seagoing fleets. From the frequency with which and the number of ships shown on Trajan’s Column in Rome, which recounts his campaigns, it is clear that they relied principally upon ships operating on the Danube and up its tributaries on both banks for their supplies. Trajan also restored the Nile-Red Sea canal and sent a fleet into the Red Sea to support his annexation of the Nabatean kingdom in AD 106. The Classis Alexandrina expanded its remit to include occasional patrols in the Red Sea, although it does not appear that any permanent naval presence was stationed there. This fleet also took over policing of the River Nile from the former Ptolemaic river force.

The reign of Hadrian (emperor AD 117–138) was marked by comparatively minor activity, the ferrying of troops by the German fleet to assist in building his wall in Britain in AD 122; an expedition to bolster the Bosporan kingdom; assisting in suppressing a revolt in Judaea. Comparative peace continued through the reign of Antoninus Pius (emperor AD 138–161). A Parthian offensive through Armenia and Syria in AD 161, led to a realignment of the eastern fleets, with the Classis Pontica moving west to Cyzicus and the Classis Syriaca being greatly strengthened.

In the latter half of the second century AD the Danube fleets were in constant action dealing with the irruptions of barbarians across the upper and middle Danube and supporting the wars of Marcus Aurelius (emperor 161–180) against them. Between AD 170 and 171 the Misene fleet operated off the Atlantic coast of Mauretania (Morocco), helping to put down tribal unrest. The end of the century saw the British fleet join a punitive expedition to Scotland (AD 185), the start of barbarian sea raiding along the northern shores of the empire and the building of forts to oppose them. It also saw another civil war from which Septimius Severus (emperor AD 192–211) emerged as emperor. The murder of his son and successor, Caracalla, in AD 217 initiated a period of instability in government with no less than twenty-two emperors and many usurpers. This period of turmoil, when barbarian pressure from beyond the borders, allied to a rejuvenated and aggressive Persian empire and coupled with internal instability and dissention, started to threaten the integrity of the empire and provided the first serious naval challenge for nearly three centuries. Further this was at a time when, once more, the Romans, lulled into a complacency by long secure and peaceful sea lanes and with a navy that had for too long been resting on its laurels, had it seems, allowed that navy, or at least large parts of it, to deteriorate. An increasing incidence of raids from across the North Sea was coupled with ventures by Goths from North of the Black Sea across that sea against Roman shores, even extending their activities into the Aegean; once again, the neglect had also permitted a resumption of that old scourge of the seaways, piracy.

Roman control of the Black Sea weakened and by mid-century Goths had occupied its northern littoral and were raiding by boat. In AD 259, a Goth fleet of as many as 500 boats, penetrated into the Aegean. They repeated the feat in AD 268 but were intercepted by Roman naval forces and twice defeated. Yet another Goth fleet attacked Thessaloniki in AD 269 but having had to abandon their siege of the city; their fleet was destroyed in a series of running battles, by the classes Alexandrina and Syriaca. In the north a system of extensive coastal defence works, allied to naval squadrons was formed along the coasts of Britain and France, to oppose increasing barbarian seaborne activity and ability. Pressure on the river frontiers taxed those fleets as the empire tottered. The navy’s formations struggled to maintain their presence in the face of declining resources and increasing enemy activity.

By the time of the accession of Diocletian (emperor AD 285–305) and the re-establishment of stability at the very end of the third century AD, a very different navy remained; the grand Imperial fleets were no more and the service was reorganised into a number of smaller squadrons, each allotted to a particular area. The formula was successful and for another century the Imperial navy, in all of its various dispositions, again kept the peace in the Mediterranean and minimised barbarian activity in the North and Black Seas although, with the probable exception of the ‘British’ fleet in the north, the service never again reached the size and strength that it had previously enjoyed. Without the large and strong central Italian fleet organisations to give it a strong separate identity, the scattered squadrons of the navy seem to have lost much of their former independent nature as each squadron was placed under the command of and seen as subordinate to the local senior military commander.

In AD 286 a local commander, Carausius, was appointed to command the Classis Britannica. He built up the fleet, improved efficiency and successfully attacked barbarian sea raiders. Noticeably the loot recovered was not always returned and the emperor Maximius (emperor AD 286–305) sentenced Carausius to death, he promptly declared himself emperor and withdrew his fleet and forces to Britain, retaining Boulogne (Gesoriacum) and Rouen (Rotomagus) in Gaul. An attempt by Maximius to end the secession in AD 288 failed when his scratch fleet was defeated by the veteran Classis Britannica. In AD 290 a treaty was made acknowledging Carausius’ position. He strengthened the economy in Britain, the fleet and further extended the shore defence system.

In AD 293 Carausius lost his holdings in Gaul, whereupon he was assassinated and succeeded by Allectus. In AD 296 the Caesar Constantius with a larger and much improved fleet, evaded Allectus’ fleet and landed troops, defeated him recovering Britain for the empire. Constantius’ ships then ranged widely, even carrying out a punitive raid on the Orkneys (Orcades). The Classis Britannica was brought back to duty and the shore defence system further extended. The result was a notable decline in barbarian activity for nearly half a century.

The role played by the river fleets on the Rhine, Danube and their tributaries when they were not engaged supporting military campaigns into barbarian lands was unrelenting. The crews had to be constantly vigilant. Not for them the luxury of spotting a ship on the horizon, and thus probably hours away; on the rivers enemy craft could emerge from a bank and be in contact in minutes. If they sailed closer to the barbarian bank, a shower of arrows could come suddenly from behind the trees and bushes; they could look upriver and see a swarm of raider’s boats cross long before they could turn and row against the current towards them. The borders had never been absolute barriers but the increasing numbers and frequency of barbarian activity severely taxed the river fleets, ultimately they would be overwhelmed.

Civil war returned in the fourth century AD and with it the first great fleet battle since Actium, between the fleets of Licinius (emperor of the East, AD 308–324) and Constantine (emperor in the West, AD 307–324; sole emperor AD 324–337) in the Dardanelles (Hellespont) in AD 323. Licinius is said to have had a fleet of 200 or more triremes, with which he sought to block the Dardanelles against Constantine’s fleet of eighty or more ‘triaconters’. This was an archaic term which, from the result, appears to have been used to describe a new, more powerful type of warship, the result being that Constantine’s fleet won a battle after which the remaining enemy ships withdrew from the war.

The empire remained reasonably strong for the rest of the fourth century AD despite the continual barbarian pressure and incursions across the river frontiers and North Sea. A change of strategic emphasis from defending the borders to having mobile forces to destroy invaders meant that more of the fighting was done on Roman territory, to its detriment, while the river forces were only expected to deal with minor incursions and only delay larger ones, their morale being affected accordingly.

There were still successes, such as in the winter of AD 357 when a band of Franks seized an old Roman river fort; the future emperor Julian (emperor AD 361–363) surrounded it and the lusoriae of the river fleet patrolled to keep the ice broken and prevent escape; the Franks surrendered. In AD 386 a huge number of Goths tried to force a crossing of the Danube and migrate to Roman territory, using a vast number of boats and rafts. The Romans concentrated their Danube squadrons, augmenting them with seagoing warships and totally destroyed the invasion.

By the end of the century, control of the North Sea had been lost and in the winter of AD 406 the Rhine froze solid, immobilizing the fleet and permitting the mass migration of barbarians into Gaul. Twenty years later the Vandals had obtained ships in Spain and crossed to North Africa. In AD 429 they took Carthage and set about creating a navy, the first non-Roman navy in the Mediterranean for four centuries. Large tracts of the western empire became barbarian kingdoms and the Rhine and British fleets disappeared. The Danube and eastern fleets remained but in the west, growing Vandal power gained them Sicily, Sardinia and Corsica. A fleet sent against them was defeated in AD 440 and they destroyed a western fleet fitting out in Spanish ports in AD 457. In that same year, the Vandal fleet was defeated at Ostia.

The final act for the Roman navy was in AD 467 when, with an eastern fleet, it drove the Vandals from Sardinia. There followed an attempt by the joint fleet to convey an army to recover Africa. The army was landed near Cape Bon but delayed their attack. Five days later, with a favourable wind, the Vandals launched an attack with fireships against the tightly packed Roman ships, anchored against a lee shore, following it up with a ram attack. They destroyed half of the Roman fleet, thwarting the invasion. It was to be the last action by a Roman fleet from the western half of the empire that would end with the abdication of its last emperor (Romulus Augustulus) in AD 476. The Eastern part of that Empire would survive for another thousand years as a body politic that for convenience, we refer to as Byzantine and would continue to operate a navy, but which is as such, part of another story.