'[I am] holding a wolf by the ears' Tiberius, emperor of Rome (r. 14-37)

The Roman empire was antiquity's largest and most powerful state. It reached its zenith under Trajan (r. 98-117), encompassing nearly 2 million square miles and containing some 60 million people. Linking its provinces were more than 250,000 miles of roads, 50,000 of which were paved. Roman engineers founded or improved more than 1,000 cities and towns, transforming the rural European landscape into a marvel of urbanization. In the third century the Roman army could field 450,000 infantry and cavalry and 45,000 sailors and marines. By the time Constantinople fell to the Ottoman Turks in 1453, the Roman army--the oldest continually existing social institution in the Western world--had been on the march for two millennia.

Rome itself was a magnificent example of cultural, technological and social superiority in its time. In 356 the city had 28 libraries, 10 basilicas, 11 public baths, two amphitheaters, three theaters, two circuses (the Circus Maximus could seat 150,000 people; the Colosseum, 50,000), 19 aqueducts, 11 public squares, 1,352 fountains and 46,602 apartment buildings. Yet little more than a century later barbarian invaders stood astride the empire's corpse, the capital in ruins.

The reasons for the empire's demise remain among the great unsettled historical debates. Regardless, it is possible to identify some of the primary forces that rendered the imperial government incapable of dealing with the lethal challenges that beset it. Of all the factors draining the empire of its ability to survive, four stand out: the changing nature of the external threat to the empire's western borders; the frequent civil wars among claimants to the imperial throne; the migration and settlement of large, armed and culturally hostile barbarian populations within the imperial borders; and the gradual erosion and eventual demise of the empire's manpower and the taxpayer base required to sustain, defend and administer the Roman state.

The Romans called the area beyond the western imperial border along the Rhine and Danube rivers the land of the barbarians. Its mostly Germanic-speaking inhabitants were relatively few in number and lived in small villages, their populations limited by their primitive agricultural technology. Employing only the wooden scratch plow, German farmers could not turn the earth sufficiently to maintain its fertility. The soil's ability to sustain adequate agricultural production quickly declined, forcing the population to move every generation or so in search of more fertile land.

The small populations and nomadism of the German tribes retarded development of their political structures. Governments were local, comprising mostly clan chiefs whose ruling power was limited by councils of advisers drawn from among other influential clan members. An individual chieftain did not have the wealth or manpower to form a warrior group loyal to him alone. Instead, clan warriors came together as circumstances required. These groups were usually small and capable only of conducting limited raids. In a few instances the tribes provided warriors to serve in limited military capacities in the Roman army itself.

The Romans...