Marcus Aurelius assumed the throne according to a previously agreed upon succession, at a time of relative peace and prosperity for Rome. Though native to Rome, his family was of Iberian (Spanish) origin; Aurelius himself was a Stoic, and given to deep philosophical thought. With his rise to power, continued security for both the Roman state and society seemed likely. His entire rule, however, was occupied by challenges that would characterize Roman dilemmas for the next two-and-a-half centuries.

Aurelius' first year as emperor (a rule shared with his adopted half-brother Lucius Verus until 169) was appropriate to his mindset of Stoicism. In 161, the Tiber River flooded, the Chatti raided, legions revolted in Britain, and the Parthians of Persia attacked, as they were unsatisfied with disadvantageous borders instituted during the reign of Emperor Hadrian (130s CE). Though the Parthians seized Armenia for a short while, Roman response came in 163-64. An army was sent east under Verus' nominal authority, but with the effective control of general Avidius Cassius. After putting the Syrian legions through rigorous re-training, he captured and burnt the Armenian capital Artaxarta, and defeated the Persians at Dura Europa on the Euphrates. Proceeding down the Tigris, he destroyed Seleucia and Ctesiphon, the latter a Parthian capital (165-6). A raid into Media beyond the Euphrates was the farthest eastern journey by Roman forces to date, and resulted in bringing western Mesopotamia into the fold as a Roman dependency. Aurelius, recognizing the need for strong eastern defenses after this war, gave Cassius supreme command of all forces from Egypt east.

Complete destruction of the Parthian threat was prevented, though. Roman units returning from battle zones brought with them a plague. Thousands of legionnaires died in the field, after which the disease spread to the cities of the Mediterranean basin, becoming "the most destructive plague in Roman history." It lasted nearly fifteen years, with an almost thirty percent mortality rate among victims. It created a manpower shortage not only in civilian sectors such as the economy, but in the military as well.

It was at just this time (165-66) that Germanic tribes began to come over the Danube River into Roman territory. Finding the Roman garrisons depleted due to the Persian campaigns and the Plague, the Germanic Marcomanni and Quadi were even able to move through the Balkans and descend upon northern Italy, reaching Aquileia. Not only raiding and withdrawing now, there were hints that they would try to settle in Roman territories. The ferocity of their attacks was a new phenomenon, and suggested that population build-up in Barbarian areas was pushing them.

Though facing military resource shortages, Marcus Aurelius was resolute in his response. Raising taxes, depleting coinage of silver content, and even selling off some of the crown jewels, he raised the necessary funds without borrowing. He then secured troops from all classes, including slaves and gladiators, and built new fortifications along the front. From 167 on, he fought on a nearly yearly basis on the border districts. After initial Roman losses, the Marcomanni were defeated in 171, while the Quadi were eliminated as a direct threat in 174. The Sarmatians were defeated the next year. Marcus' planned offensive across the Danube was prevented in 175, however, by insurrection. Avidius Cassius, with control over eastern armies, had himself proclaimed emperor based on a rumor of Aurelius' death. Though Marcus Aurelius successfully suppressed the revolt, it was not until 178 that he was able to pursue the Quadi over the Danube into Bohemia. He was planning to advance the Roman border east and north to the Carpathian Mountains and Bohemia when he became ill and died in 180.

Aurelius died with an heir, the first Emperor to do so since Vespasian. Commodius, however, was the opposite of his father. Devoted to enjoyment and life in a fantasy world of self-adoration and athletics, he began his reign by forming a treaty with the Germanic tribes that did not reflect his father's successes. Barbarian prisoners were returned, and Rome agreed to pay a subsidy to the tribes to keep them away. For the next twelve years, Commodius tried to enjoy himself as much as possible. Retiring from public life except for sporting events in which he competed, he allowed courtiers and favorites to run the government. Administration quickly diminished, with bureaucratic offices actually going for sale. In 192, when he tried to appear in public as both a gladiator and a consul, he was killed by members of the Praetorian Guard, the elite Palace guard. The Praetorian Prefect, Laetus, then selected as emperor Helvius Pertinax, who had been a close adviser to M. Aurelius. The Senate accepted the nomination. Pertinax began his reign in a serious tone, rehabilitating depleted royal finances, taking the title princeps as opposed to the more imposing imperator, and insisting on stricter terms of military service. The Praetorians tired of him quickly, though, and murdered him in 193. Beginning a process that only grew in the next decades, the guards chose a successor, who was not universally acclaimed. Various imperial legions proclaimed their own commanding generals as Emperor, and up to four claimants fought out the succession.

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