THE EARLY ROMAN EMPIRE (31 BC-AD 193) 

After Actium and on two other occasions, Augustus solemnly closed the gates of the shrine of Janus (a gesture of peace) to show that Rome had peace as well as a princeps. These well-publicized gestures were purely temporary; the gates were swiftly reopened. His proconsular imperium made Augustus the arbiter of peace and war, and an ostensible search for defensible frontiers made his a very warlike reign. While the republic had left the limits of Roman territorial claims rather vague and indefinite, he planned conquests stretching to the boundaries defined by nature (deserts, rivers, and ocean shores), not always, however, with immediate annexation in mind. When annexation did occur, it was followed by the construction of solidly built military roads, paved with thick stone blocks: these also served the official post system (cursus publicus) and were provided with rest stages and overnight lodges at regular intervals. 

Areas where subjugation looked arduous and where Romanization seemed problematic were left to client kings, dependent on the emperor's support and goodwill and under obligation to render military aid to Rome. Such satellite kingdoms spared Augustus the trouble and expense of maintaining strong defenses everywhere; nevertheless, their ultimate and intended destiny was incorporation as soon as it suited their overlord's convenience. Usually, territory was gained more easily by creating and subsequently incorporating a client kingdom than by launching an expansionist war. 

In the south, Augustus found suitable frontiers quickly. In 25 BC an expedition under Aelius Gallus opened the Red Sea to Roman use and simultaneously revealed the Arabian Desert as an unsurpassed and, indeed, unsurpassable boundary. The same year Gaius Petronius, the prefect of Egypt, tightened Rome's grip as far as the First Cataract and established a broad military zone beyond it. The vast region north of the Sahara and the Atlas Mountains was also secured (c. 25) after a series of punitive raids against native tribes and the annexation of one client kingdom (Numidia) and the creation of another (Mauretania). Three legions, two in Egypt and one in Africa (a senatorial province), policed the southern shore of the Mediterranean. 

In the west, consolidation was extended to the Atlantic. Gaul, Julius Caesar's conquest, was organized as four provinces: senatorial Narbonensis and the imperial three Gauls (Aquitania, Belgica, and Lugdunensis). In Spain, after Agrippa successfully ended in 19 BC the last campaign that Augustus had launched in person in 26, three provinces were formed: senatorial Baetica and imperial Lusitania and Tarraconensis. Three legions enforced Roman authority from Gibraltar to the mouth of the Rhine. Augustus ignored the advice of court poets and others to advance still farther and annex Britain. 

In the east, Parthia had demonstrated its power against Crassus and Antony, and Augustus proceeded warily. He retained Antony's ring of buffer client kingdoms, although he incorporated some, including the most celebrated of them, Judaea; he made it a province in AD 6, respecting, however, some of the customs of its Jewish inhabitants. Augustus stationed four legions in Syria and obviously envisaged the Euphrates River and the northern extension of the Arabian Desert as the desirable frontier with Mesopotamia. Farther north, however, no such natural line existed. North of the Black Sea the client kingdom of the Cimmerian Bosporus, under its successive rulers Asander and Polemo, helped to contain southward and westward thrusts by the Scythians, an Iranian people related to the Parthians, and this provided protection in the north for Anatolia and its provinces (senatorial Asia and Bithynia-Pontus and imperial Cilicia and Galatia, the latter a large new province created in 25 BC out of Amyntas' client kingdom). By a show of force, Augustus' stepson Tiberius, in 20 BC, recovered the standards lost at Carrhae and installed Tigranes as client king of Armenia. Although Augustan propaganda depicted this as a famous victory, strategic considerations inevitably obliged the Parthians, once they settled their internal, dynastic dissensions, to dispute Roman control of Armenia. Thus it can hardly be said that Augustus settled the eastern frontier. Missions were sent to the East repeatedly (Agrippa, 17-13 BC; Gaius Caesar, AD 1-4; Germanicus, 18-19), and Armenia remained a problem for Augustus' successors: Tiberius successfully maintained Roman influence there, but Gaius and Claudius failed to do so, leaving Nero with a difficult situation. 

In the north, too, there was difficulty. The Alps and their passes were finally subjugated early in Augustus' reign. This enabled Tiberius and his brother Drusus between 16 and 8 BC to conquer all the way to the great rivers of central Europe. New provinces were created in the Alps and Tyrol (Maritime and Pennine Alps, Raetia, Noricum) and also farther east (Pannonia, Moesia). Stability along the Danube was precariously maintained, under Augustus and later, by means of periodical alliances with Maroboduus and his successors, who ruled Germanic tribes such as the Marcomanni and Quadi in Bohemia to the north of the river, and by the existence of a Thracian client kingdom to the south of its lowest course. The push across the Rhine began in 12 BC; although it reached the Elbe, consolidation beyond the Rhine proved elusive. A revolt in Pannonia (AD 6-9) interrupted it, and, in AD 9, German tribes under Arminius annihilated Quinctilius Varus and three legions in the Teutoburg Forest. This disaster reduced the number of legions to 25 (it did not reach 28 again until half a century later), and it disheartened Augustus. Old and weary, he withdrew to the Rhine and decided against all further expansion, a policy he urged upon his successor. For the watch on the Rhine the military districts of Upper and Lower Germany were created, containing eight legions between them. Another seven garrisoned the Danubian provinces. These figures reveal imperial anxiety for the northern frontier.
