The Pax Romana, or Roman Peace, is a Latin term referring to the Empire in its glorified prime. From the end of the Republican civil wars, beginning with the accession of Augustus in 27 BC, this era in Roman history lasted until 180 AD and the death of Marcus Aurelius. Though the use of the word 'Peace' may be a bit misleading, this period refers mainly to the great Romanization of the western world. The Roman legal system which forms the basis of many western court systems today brought law and order to the provinces. The Legions patrolled the borders with success, and though there were still many foreign wars, the internal empire was free from major invasion, piracy or social disorder on any grand scale. The empire, wracked with civil war for the last century of the Republic, and for years following the Pax Romana, was largely free of large scale power disputes. Only the year 69 AD, the so-called 'Year of the Four Emperors', following the fall of Nero and the Julio-Claudian line, interrupted nearly 200 years of civil order. Even this was only a minor hiccup in comparison to other eras. The arts and architecture flourished as well, along with commerce and the economy.
For the enemies of Rome, however, the Pax Romana didn't signal peace or tranquility. The legions saw considerable action bringing previously un-pacified territory under control, spreading Roman influence in the Balkans, and attempting expansion in Germania. The last would end up one of the great disasters in Roman military history, however, and would signal a complete shift in foreign policy that would remain largely in effect (excepting the Claudian invasion of Britain) until the time of Trajan (early 2nd century AD).
Under Augustus, it was important for new emperor to establish border security, and find new pursuits for 150,000 active legionaries. Immediately after, and during his reorganization of the legions, Augustus did just that. In the region of the world now known as the Balkans, roughly corresponding to ancient Dalmatia, Illyricum, Pannonia, Moesia and Thracia, Marcus Licinius Crassus, grandson of the first Triumvir, conducted an extensive campaign from his province of Macedonia. Following the campaigns of Octavian in the previous decade, Crassus, followed up that success by pushing the frontier borders as far north as the Danube, pacifying the local Pannonians and Illyrians. His success, and service to Augustus were short-lived, however, and Crassus was soon removed from his position for demanding triumphal honors (when they rightfully belonged to Augustus under the new imperial era). Within a few years, Celtic invasions in 16 BC roused the Pannonians and military action was required again.
Augustus' stepson Tiberius undertook his first major campaign in 15 BC to bring the region to order. Initially successful, Tiberius was transferred to the region just north of the Alps, along with his brother Drusus, to quell disturbances that may or may not have been connected to the problems in the Balkans. While Tiberius and Drusus were bringing Roman order there, eventually establishing the provinces of Raetia and Noricum, roughly made up of modern Switzerland and Western Austria respectively, the Pannonians were still not settled. In 13 BC, a new revolt broke out and Augustus sent Agrippa to handle the situation. However, Agrippa soon succumbed to illness in the following year, not only shaking up the early establishment of Augustus' dynastic plans, but forcing the return of Tiberius to command. Here Tiberius would remain for the next three years (12 to 9 BC) subduing the region once again. In so doing, he established a new province of Pannonia, and sometime before 6 AD, the province of Moesia. The tactics used by Tiberius to quell the population, selling off young men and children into slavery, though effective would eventually have terrible repercussions; but for the time being order prevailed and Tiberius was moved to Germania.
Despite maintaining nominal control of Hispania for the better part of two centuries, resistance continued, especially in the northeast (where the all important gold mines of Gallica were located). After conducting a census of Gaul, immediately after his victory in the civil war, Augustus moved on to Spain to eliminate this resitance. Though mostly successful in his campaigns, revolts continued to break out over the next twenty years. Though these were relatively minor seemingly last ditch efforts to throw off the Roman yolk, the final subjugation wasn't completed until 13 BC. At this point Hispania was divided into three provinces: Baetica, Lusitania and Terraconensis; and it became a perfect example of the Romanization process.
The East provided problems for the new empire as well. After touring the eastern provinces, previously under the control of Antony, the situation was mostly settled, but there were issues to be dealt with. In Egypt, the personal property of the emperor, Augustus first had trouble with his own Prefect, C. Cornelius Gallus. After subduing a revolt in 30 BC, Gallus disgraced himself by inscribing his own glory on various public works and the Pyramids, forcing Augustus to recall him. Cornelius soon committed suicide and was replaced by M. Aelius Gallus. Aelius launched a poorly fated attack on the neighboring tribes in Arabia, and left Egypt open to Ethiopian raids. Soon replaced, C. Petronius brought order back to Egypt, but was unable to subdue the Ethiopian forces of the Kingdom of Meroe. Ending in a stalemate, Augustus, and successive emperors, left the southern border of Egypt as it was and maintained a fairly strict policy of border protection rather than expansion. Trouble with the ascension of Celtic client kings in the inland (now central Turkey) region of Galatia forced Augustus to annex the entire province in 25 BC. While this occurred without major incident it also brought the eastern Roman borders ever closer to Parthian rivals.
Since the destruction of Marcus Licinius Crassus at Carrhae in 53 BC, Rome and Parthia maintained an uneasy peace. Caesar had planned a major expedition to avenge this defeat, but his assassination put an end to it. Antony launched his own campaigns, without success, while master of the East, and the task was thus left to Augustus. The main points of contention between the two powers were the kingdom of Armenia and its shifting loyalties, the issue of captured Roman standards still in Parthian possession and the presence of the Parthian King's son as a hostage in Rome. In 21 BC, Augustus arranged for the return of Crassus' lost legionary standards in exchange for the King's son. Though the matter would require the presence of both Augustus (with an army in Syria) and Tiberius moving east from Asia Minor, the exchange was arranged without bloodshed. Despite the lack of true military achievement, Augustus celebrated a triumph and the return of the standards was immortalized on imperial coinage. However, Armenia continued to be an issue and would really remain so late into the Empire. In 2 BC Augustus sent Gaius Caesar, his grandson and eventual heir, to deal with the problem, but he was wounded in a minor battle. While Armenia would not become an official province until Trajan's reign, it did shake up the imperial dynasty. After a long illness resulting from his wound, Gaius died, as did his brother Lucius shortly thereafter, leaving Tiberius as the heir to the throne.
Judaea too was annexed as a province in 6 AD. After the death of King Herod in 4 BC, the kingdom was divided among his sons, but this proved an inadequate form of government to Rome. Replaced by a Praefect, direct Roman rule was anything but representative of the 'Pax Romana'. Throughout its early occupation, Jewish natives resisted fervently. Though during Augustus reign, this resistance was minimal, over the course of the next century Judaea was anything but peaceful. The religious emergence of the Christian cult would eventually shake the very fabric of the Roman social system, but helped create its own problems with the Jewish hierarchy early in Tiberius' reign. Later, massive revolts evolved into a major war under the Flavian emperors (Vespasian and Titus) before Judaea was finally pacified in the 60's AD.
Back in the west, in Germania, Tiberius brother Drusus was responsible for campaigns into the Germanic interior, while Tiberius was operating in Pannonia. Drusus successfully pushed the Roman border from the Rhine all the way to the Elbe but his death in 9 BC forced the transfer of Tiberius to take over. Shortly after, however, Tiberius surprisingly retired from public life and the details of the Germanic campaigns conducted by his replacements are almost non-existent. Though some level of military success was certainly achieved in the interim, the written history was likely recorded in such a way as to glorify Tiberius and not those who replaced him. Tiberius returned to command in 4 AD and set about ousting the Marcomanni (one of the major enemies in the Germanic wars of Marcus Aurelius), from territory that would smoothly connect the new provinces of Raetia and Noricum to the previous German conquests.
While on campaign, however, word reached Tiberius of a massive uprising back in Pannonia. Forced to abandon the Germanic campaign, he returned to Pannonia where fighting would continue until 9 AD. Though this would be the last major revolt in the Balkan province, its toll was felt all the way back in Rome. As a result of the numerous foreign wars, Augustus was finding new recruits difficult to find. In a terribly unpopular, yet necessary move, he was forced to start drawing recruits from freedmen and even purposely freed slaves, just for this purpose. If the strain on Roman manpower was beginning to be pushed to the extreme already, events were about to unfold that would force Augustus to call for a complete end to Imperial expansion.