Naval warfare in the age of migration and invasion reflected the patterns of the age. While most of the threats to Afro-Eurasian civilizations came from the horsed nomads of the Asian steppes, some migratory peoples moved by sea. As a result, the scope of naval warfare spread beyond the confines of the Mediterranean world, whose comparatively benevolent seas had fostered the ancient world’s only real arena of naval conflict. New patterns of trade in the more fragmented postclassical world also stimulated maritime activity. For both these reasons, naval warfare assumed greater importance globally than it had in the previous age.
Naval operations between 400 and 1100 may be analyzed in terms of two major types, or models: imperial defense and the activity of predatory sea peoples. Most of the naval powers of the age fell into one of the two categories, but the models represent poles on a continuum, and some naval forces were hybrids of the two.
Navies of imperial defense were parts of unified, centrally directed defensive strategies. They were supported by bureaucracies that saw to the recruitment of sailors and marines, the building and repair of ships, and logistical support for fleets. In other words, navies of imperial defense were under state direction. Naval defense in this model was thus linked to trade and general maritime activity through the government: Taxes on trade often funded the navy, and maritime resources of ships and experienced sailors moved from the merchant to the military sphere via government direction or appropriation. Technologically, navies of imperial defense tended to be sophisticated—in fact, they tended to be among the most technologically advanced products of their civilizations—and to be well adapted to the particular marine geography and climate in which they operated. The tactical aim of such navies was, broadly, to sink enemy ships, a task for which their technology was also well developed.
The Byzantine Empire is the dominant example of this model of naval activity. Near the end of the period, Song China also created such a navy.
The characteristics of predatory sea peoples contrast sharply in each case with imperial navies. Naval activity in this model was generated spontaneously from the socioeconomic structure, rather than being a creation of government policy. It was mainly economic and offensive in its goals—that is, it was aimed at predatory plunder. Rather than the fleet or formal squadron, the boatload was the basic unit of military, and indeed social, organization in this model: A leader and his followers used their own boat for their own purposes. Naval activity among predatory sea peoples is tied to state formation and political structure, but in the context of weak central authority. Thus, much of the activity of such naval powers was private or semiprivate, rather than being under state direction in support of a clear policy. Technologically, such naval forces were less sophisticated, and certainly much less specialized, than imperial navies. Though capable of impressive accomplishments, their ships did not press the limits of the age’s technical know-how. The tactical aim of predatory naval forces, if it came to a fight at sea (which was less likely than for imperial navies, which sought such encounters), was not to sink but to capture enemy ships. Ships were valuable prizes, of significant economic worth in themselves, as tools for further raiding, and also likely to contain rich booty in goods and in people who could be sold as slaves. But the more common tactical (and strategic) aim of such forces was to raid unguarded shores. The key examples of this model were the Vikings of northwest Europe, the Cholas of southern India, and Srivijaya in the Straits of Malacca in Southeast Asia.
Two important naval powers in the Mediterranean— the Muslims and later the Italian city-states—were in varying ways hybrids of the two models. For them, naval activity was a combination of predation and imperial offensive (as well as defensive) strategy. For these powers, the state played a role closer to the imperial model, but often in a more fragmented and limited political context. That is, aside from the Caliphate at the height of its powers, these were strong but small states. Some, such as Venice, were, in effect, parasitic on an imperial power. Technologically, such powers proved to be skillful at adapting, adopting, and innovating technologically, though with less of a specialized military focus than the imperial powers. The Muslim world, especially, with one oar in the Mediterranean and one oar (or sail) in the Indian Ocean, would become a breeding ground for maritime technological advances. The tactical aim of these naval forces varied with circumstances but was likely to lean to capture rather than sinking, since for small states, even strong ones, a ship was likely to be a valuable resource.
These models provide us with a framework for examining the naval powers of the age in more detail. But for all the naval powers of this age, naval activity was still, as in the ancient Mediterranean, essentially amphibious. That is, a maritime technology limited fleets largely to coastal waters, and control of the seas was, in the modern sense, an impossibility.