[Sidenote: Change in the imperial policy.]

Under the emperors, the whole policy of the government was changed. They
no longer thought of further aggrandizement, but of retaining the
conquests which were already made. And if they occasionally embarked in
new wars, those wars were of necessity rather than of ambition, were
defensive rather than aggressive. New provinces were from time to time
added, but in consequence of wars which were waged in defense of the
empire. The conquest of Britain and Judea was completed, and various
conflicts took place with the Germanic nations, who, in the reign of
Antoninus, formed a general union for the invasion of the Roman world.
These barbarians were the future aggressors on the peace of the empire,
until it fell into their hands. The empire of Augustus may be said to
have reached the utmost limits it ever permanently retained, extending
from the Rhine and the Danube to the Euphrates and Mount Atlas,
embracing a population variously estimated from one hundred to one
hundred and thirty millions.

[Sidenote: Perfection of military art.]

When Augustus became the sovereign ruler of this vast empire, military
art had reached the highest perfection it ever attained among any of the
nations of antiquity. It required centuries to perfect this science, if
science it may be called, and the Romans doubtless borrowed from the
people whom they subdued. They learned to resist the impetuous assaults
of semi-barbarous warriors, the elephants of the East, and the phalanx
of the Greeks. Military discipline was carried to the severest extent by
Marius, Pompey, and Caesar.