Composition of Armies and Distribution of Troops in the Third Stage.—If the system of legions and auxilia in the early Empire was novel, the use made of them was no less so. The later Repub lic offers to the student the spectacle of large field armies, and though it also reveals a counter tendency to assign special legions to special provinces, that tendency is very feeble. Augustus ended the era of large field armies : he could not afford to leave such weapons for future pretenders to the throne. By keeping the Em pire within set frontiers, he developed the counter tendency. That policy exactly suited the military position in his time. The early Roman empire had not to face—like modern empires—the danger of a war with an equal enemy, needing the mobilization of all its national forces. From Augustus till A.D. 25o Rome had no con terminous foe from whom to fear invasion. Parthia, her one and dangerous equal, was far away in the East and little able to strike home. Elsewhere, her frontiers bordered more or less wild bar barians, who might often harass, but could not do serious harm. To meet this there was need, not of a strong army concentrated in one or two cantonments, but of many small garrisons scattered along each frontier, with a few stronger fortresses to act as mili tary centres adjacent to these garrisons.
Accordingly, a system grew up under Augustus and his imme diate successors whereby the whole army was distributed along the frontiers or in specially disorderly districts (such as north west Spain) in permanent garrisons. On the actual frontiers and on the chief roads leading to them were numerous cohorts and alae of auxiliaries, garrisoning each its own castellum of 3-7 acres in extent. Close behind the frontiers, or even on them, were the 25 legions, each (with a few exceptions of early date) holding its own fortress (castra stativa or hiberna) of 50-60 acres. Details varied at different times. Sometimes, where no Rhine or Danube helped, and where outside enemies were many, the frontier was further fortified by a continuous wall of wooden palisades (as in part of Germany; see LIMES) or of earth or stone (as in Britain, see BRITAIN : Roman), or the boundary might be guarded by a road patrolled from forts planted along it (as in part of Roman Africa). The result was a long frontier guard covering Britain and Europe from the German ocean to the Black sea, and the upper Euphrates valley, and the edge of the Sahara south of Tunis and Algeria and Morocco, while the wide Empire within saw little of its soldiers.
The following table shows the disposition of the legions about A.D. I 20 and for many decades subsequently. It would be impossi ble, even if space allowed, to add the auxiliaries, as the details of their distribution are too little known. But as the number of
auxiliaries in any province was probably rather greater than the number of legionaries, the sizes of the various provincial armies can he calculated roughly. Thus Britain was held probably by men. Each provincial army was commanded either by the governor of the province or (in a few exceptional cases) by the senior legatus of the legions stationed there:— The total of legionaries may be put at about 180,000 men, the auxiliaries at about 200,000. If we exclude the "household" troops at Rome, the police fleets on the Mediterranean and the local militia in some districts, we may put the regular army of the Em pire at about 400,000 men. This army, as will be plain, was framed on much the same ideas as the British army of the 19th century. It was meant not to fight against a first-class foreign power, but to keep the peace and guard the frontiers of dominions threatened by scattered barbarian raids and risings. Field army there was none, nor any need. If special danger threatened or some special area was to be conquered—such as southern Britain (A.D. detachments (vexillationes) were sent by legions and sometimes also by auxiliaries in adjacent provinces, and a field force was formed sufficient for the moment and the work.