For the logical issue of a thorough-going imperial policy--one in which the possession of physical ability may be presupposed--is the formation of an universal empire. And, in fact, the two most powerful and ardent imperialists of antiquity, Alexander the Great and Julius Cæsar, aimed to include in their dominions the entire inhabitable world. This issue was, however, never more nearly reached than in the long period before and after the Christian era during which only shifting nomads and intractable Parthians disputed successfully the will of the Roman Senate and the orders of the Roman emperors. For five hundred years after the triumph of Constantine the universality of the Roman empire was as mandatory in men's thinking as was the catholicity of the Christian Church. "There are many 'empires' in the world to-day," says Professor Bury[1] in explaining the coronation of Charlemagne in 800 A.D., "but in those days men could only conceive of one, the Roman _imperium_, which was simple and indivisible; two Roman empires were unimaginable. There might be more than the one emperor; but these others could only be legitimate and constitutional if they stood to him in a collegial relation." How thoroughly the Romans impressed the concept of universality upon the term empire may be judged by the fact that, in the face of all realities, the Frankish monarchs at Aachen and the Greek kings at Constantinople ruled as colleagues a Roman empire which stretched from the borders of Armenia to the shores of the Atlantic. Transcendent as is the imperial achievement of the Romans, and unrivaled as is the political sagacity with which they consolidated their power and made it enduring, it must still be recognized that they were the heirs, in war, diplomacy, and government, of the Greeks, their predecessors. They worked with greater power and with larger units than did the Spartans and the Athenians. They benefited by the brilliant inventions and the costly errors of the Macedonians whose kingdoms they destroyed. But their success simply brought to a culmination the imperial movement in which Sparta, Athens, and Macedon were worthy co-workers. It is our task in this series of essays to examine in turn the imperial experiments by which the Greeks not only won a field for the display of their own talents, but also prepared the way for the unification of the ancient world in the empire of Rome. * * * * *