long, were destined to be broken and the barbarian deluge from the north and east would pour over the Empire. [Illustration: FIG. 31. A BODYGUARD OF GERMANS A relief from the Column of Marcus Aurelius, at Rome, erected to celebrate his victories over the Marcomanni, and other German tribes.] THE BOUNDARIES OF THE EMPIRE ARE BROKEN. While temporary extensions of territory had at times been made beyond the Rhine and the Danube, these rivers had finally come to be the established boundaries of the Empire on the north, and behind these rivers the Teutonic barbarians, or _Germani_, as the Romans called them, had by force been kept. To do even this the Romans had been obliged to admit bands of Germans into the Empire, and had taken them into the Roman army as "allies," making use of their great love for fighting to hold other German tribes in check. In 166 A.D. the plague, brought back by soldiers returning from the East, carried off approximately half the population of Italy. This same year the Marcomanni (see Figure 18), a former friendly tribe, invaded the Empire as far as the head of the Adriatic Sea, and it required thirteen years of warfare to put them back behind the Danube. Even this was accomplished only by the aid of friendly German tribes. From this time on the Empire was more or less on the defensive, with the barbarian tribes to the north casting increasingly longing eyes toward "a place in the sun" and the rich plunder that lay to the south, and frequently breaking over the boundaries. Rome, though, was still strong enough to put them back again.