and Severus
increased it to thirty-three, totalling over 180,000 men. A corresponding
increase had been made in the numbers of the auxiliaries. From about
150,000 in the time of Augustus they had increased to about 220,000 in the
second century. The total number of troops in the Roman service at the
opening of the third century was therefore about 400,000; one of the
largest professional armies the world has ever seen.

*The system of frontier defence.* A second momentous fact in the military
history of the principate was the transformation of the army from a field
force into garrison troops. This was the result of the system developed
for the defence of the frontiers. Augustus, for the first time in the
history of the Roman state endeavored to preclude the possibility of
indefinite expansion by attaining a frontier protected by natural barriers
beyond which the Roman power should not be extended. Roughly speaking
these natural defences of the empire were the ocean on the west, the Rhine
and the Danube on the north, and the desert on the east and south. At
strategic points behind this frontier Augustus stationed his troops in
large fortified camps, in which both legionaries and auxiliaries were
quartered. These camps served as bases of operations and from them
military roads were constructed to advantageous points on the frontier
itself to permit the rapid movement of troops for offensive or defensive
purposes. Such roads were called _limites_ or "boundary paths," a name
which subsequently was used in the sense of frontiers. These _limites_
were protected by small forts manned by auxiliary troops.

*The fortification of the limites.* Although Claudius and Vespasian
discarded the maxims of Augustus in favor of an aggressive border policy
they adhered to his system for protecting their new acquisitions in
Britain and the Agri Decumates. However, these conquests and that of the
Wetterau region by Domitian pushed the frontier beyond the line of natural
defences and led to the attempt to construct an artificial barrier as a
substitute. It was Domitian who took the initial step in this direction by
fortifying the _limites_ between the Rhine and Main, and the Main and the
Neckar, with a chain of small earthen forts connected by a line of wooden
watchtowers. To the rear of this advanced line there were placed larger
stone forts, each garrisoned by a corps of auxiliaries, and connected by
roads to the posts on the border. While