In modern times, we think of great navies patrolling the oceans of the world. The British Empire, for example, owes the advent of its naval superiority to its victory over the Spanish Armada and the subsequent focus on providing protection for its trading partners and colonies. In Roman times, the naval landscape was principally the Mediterranean Sea, or as the Romans liked to call it, Mare Nostrum.
As I have discussed in other posts, the Romans came late to the game of sea trade and naval power following the successes of the Phoenicians, Greeks, and Carthaginians. They had no navy until the first Punic War (264 B.C.) when it became an important instrument for the capture of Sicily. By the time the empire began, naval power was a critical element of Roman strategy.
A remarkable yet mostly obscure part of the Roman naval story concerns the fleets of the inland frontier. I know of no other case in history where a large scale power deployed a navy for the control of rivers. By accident or design, the northern boundary of the empire would be marked by the great rivers of Europe; the Rhine and the Danube, so a naval force was required to act in support of the army. The Rhine was the western boundary of the empire from the time of Julius Caesar with the Danube following during the time of Augustus, who sought to move the boundary north to avoid attacks emanating from the Alps. Augustus had Drusus and his brother Tiberius push east from the Rhine as far as the Elbe, but following reverses like the Massacre at Teutoburg, Rome retreated to its
old boundary on the Rhine.
With rivers as a physical boundary, Rome needed a naval force to protect it: protect merchant traffic, quickly ferry the army to vulnerable locations, and control any adversarial movement on the waterways. Below is a map of the Rhine and Danube Rivers showing where the Roman fleets were located.
The Iron Gates are a physical landmark in the Lower Danube which divides the river. Its waterfalls and rapids made ship navigation during antiquity problematic, and it was not until 1831 that a successful channel was dug. See the photograph below.
The Roman solution to this Iron Gates problem was to deploy two navies. Classis Moesica on the eastern side and Classis Pannonica on the western side. These fleets were responsible for supporting the volatile Balkan frontier. The upper Danube, running from the Alps through modern Hungary was a more stable boundary which did not require a naval force during the first two centuries AD. A third naval force, the Classis Germanica was based near modern Bonn and was responsible for the Rhine region.
These Roman frontier fleets used a smaller class of ships than those used in the larger bodies of water, principally the Liburna, which was essentially a bireme with two rows of oars, possibly similar to the old Greek pentekontor. The crew was organized as a century like the army but not subdivided into cohorts because there was no tactical movement necessary at sea. A centurion was responsible for training the crew and took orders from a Praefect who acted as an administrator for the navy. These men were of high standing usually members of the equestrian class. The naval administration linked ships and sailors directly to Caesar rather than acting through the army chain of command. This peculiarity was an artifact of the time of Augustus when he placed so much trust in Agrippa as his naval commander and then assumed personal control following the death of his friend.
Actions undertaken by the northern fleets were sporadic and depended on the whether there were pressing threats to the frontier. For example, there were significant actions from 20 B.C. to 10 A.D. under Augustus when he was trying to establish provinces in the Balkans, but peacetime idled the navy from 15-69 A.D. After 85 A.D, the Dacian Wars occupied the fleets of the lower Danube for twenty years.
There are no real descriptions of the northern fleets in battle, only their support of the efforts of the army. Lack of historical detail and the fact that the indigenous people lacked navies, leads one to believe there few independent actions that one could label noteworthy.