itutions of political and intellectual freedom that have made
western civilization mistress of the world. For this triumph of
West over East, of Christianity over barbarism, we have to thank
partly the courage and genius of great warriors and statesmen who
arose here and there, like Alfred of England and Martel of France,
but chiefly the Eastern Empire, with its capital at Constantinople,
which stood through this entire epoch as the one great bulwark against
which the invasions dashed in vain. In this story of defense, the
Christian fleets won more than one Salamis, as we shall see in
the course of this chapter.

In the year 328 A.D. the Emperor Constantine the Great moved his
capital to Byzantium and named it "New Rome." In honor of its founder,
however, the name was changed soon to "Constantinople," which it
has retained ever since. It may seem strange that after so many
glorious centuries Rome should have been deprived of the honor of
being the center of the great empire which bore its own name, but
in the fourth century the city itself had no real significance.
All power rested in the person of the Emperor himself, and wherever
he went became for the time being the capital for all practical
purposes. At this time the empire was already on the defensive and
the danger lay in the east. Constantine needed a capital nearer
the scene of future campaigns, nearer his weakest frontier, the
Danube, and nearer the center of the empire. Byzantium not only
served these purposes but also possessed natural advantages of a
very high order. It was situated where Europe and Asia meet, it
commanded the waterway between the Black Sea and the Mediterranean,
and it was a natural citadel. Whoever captured the city must needs
be powerful by land and sea. Under the emperor's direction the new
capital was greatly enlarged and protected by a system of massive
walls. Behind these walls the city stood fast for over a thousand
years against wave after wave of barbarian invasion.

Of the wars with the Persians, the Vandals, and the Huns nothing
need be said here, for they do not involve the operations of fleets.
The city was safe so long as no enemy appeared with the power to
hold the sea. That power appeared in the seventh century when the
Arabs, or "Saracens," as they were called in Europe, swept westward
and northward in the first great Mohammedan invasion.

Most migrations are to be explained by the pressure of enemies,
or the lack of fo