From the outset of his succession of Antoninus Pius, Marcus Aurelius was confronted by restless Germanic (chiefly Marcomanni and Qaudi) tribes north of the Danube. However, pressing matters from Parthia in the east required far more urgent intervention. Initially, the Germanic issues were handled by provincial governors and an uneasy peace was reached but while there is little surviving evidence of the reasons for the Marcomannic unrest (other than rather nondescript suggestions of migratory tribes), the situation steadily destabilized throughout the early reign of Aurelius.
Early in AD 169 outright war with the Marcomanni and Quadi finally broke out. They crossed the Danube en masse overrunning limited defenses between them and the Italian mainland. For the first time since the invasions of the Cimbri and Teutones some two and a half centuries earlier, a Germanic invasion once again threatened the interior of the empire. The joint emperors (Lucius Verus having recently returned from the Parthian campaigns) acted quickly to alleviate the threat and used extreme measures to do so. With the war in the east and the growing plague brought back by victorious legions and the recent famine caused by the flooding of the Tiber, the Romans were in dire need of manpower. Perhaps illustrating how grave the situation was, Marcus Aurelius turned to rather non traditional means of replenishing the military ranks. He recruited and armed ‘volunteer’ slaves as well as gladiators and Germanic auxilia. (From the term voluntarii which was associated with the recruitment of the Volones in the second Punic War against Hannibal. This term evolved to mean the recruitment of any slaves or non citizens for military service.) Later these emergency recruits were to be supplemented by the formation of Legio I and II Italica.
While the Germanics besieged the important port of Aquileia on the coast of the Adriatic Sea, the freshly assembled Romans led by their joint emperors marched north to meet them. Perhaps an indication that the invasion was in part prompted by a perceived weakness of the Roman military machine, as word spread of the Roman advance the Germanics quickly marched back to the Danube seeking conciliatory terms. Marcus Aurelius intended to continue pressing north of the Alps, but the untimely death of Lucius Verus forced a brief respite to the offensive plans. Marcus returned to Rome to oversee the funeral of his adopted brother and son-in-law but soon returned north to settle matters more permanently. While the bulk of the Germanic campaigns were to be largely successful, the duration of the emperors presence (for the better part of his reign from AD 169 to 180 interrupted only briefly by an eastern revolt) was also an indication of the difficulty in the task.
The objective was a permanent peace along the Danube and perhaps the incorporation of new provinces to administer this peace. The Historia Augusta suggests that during the last years of the war Marcus Aurelius “waged war with the Marcomanni, the Hermunduri, the Sarmatians, and the Quadi, and had he lived a year longer he would have made these regions provinces.” As was the norm in Roman campaigns against ‘barbarians’ the strategy of divide and conquer was employed. It is difficult to determine what ultimate effect the campaigns could have had, despite the understanding that the Danube was secured from immediate threat for almost another century. If not for the revolt of one of the victorious legates of the Parthian war, Avidius Cassius, perhaps the Danubian settlement may have been even more effective than it was at first glance.
In the midst of the Marcommanic War circa AD 175, the governor of Syria Avidius Cassius claimed the empire for himself. The circumstances are unusual. Cassius had already been granted extraordinary power, or imperium, in the east likely stemming from Marcus Aurelius’ occupation with the Germanic wars. At this time there was a rumor of Marcus Aurelius’ death and Cassius seized an opportunity to wrest control from his certain heir, the 14 year old Commodus. There were also rumors that Cassius had been a lover of the empress Faustina, and that she was instrumental in encouraging Cassius upon the false news of her husband’s death. Whether Cassius made a mistake in his assessment of the news or spread the rumor himself in a bid for power, we will likely never know, but the very much still living emperor responded quickly. He abandoned the Danubian campaigns and began the march east, but news of his impending arrival preceded him. Avidius Cassius and his fellow conspiring officers were killed by men loyal to the true emperor. Marcus Aurelius displayed the sort of stoicism that was his reputation and let the matter die with this act. He spent some time in the east confirming loyalty and settling provincial matters, but the revolt was little more than a distraction from the goal of settling the Germanic affair. However, as the situation unfolded the distraction proved to be enough to prevent the completion of the emperor’s goals.
Marcus Aurelius returned to the Danube after stops in Greece and Rome, where his son Commodus (now 16 years old) was not only confirmed as his father’s heir but was granted joint imperial power. Back along the northern frontier the joint emperors conducted the campaigns in earnest between AD 177 and 180. The Marcomanni, the Hermunduri, the Sarmatians, and the Quadi among several other lesser tribes were defeated in turn. Sarmatian cavalry was incorporated as auxilia and sent to Britannia where they would serve the empire well into the 5th century. However, by 180 AD Marcus Aurelius died at the age of 59, perhaps from years of exposure to plague. Before his death he urged his son to continue the war through to its completion (which according to the ancients may not have taken more than another year) but after 11 years of war and a debilitating plague, Commodus instead established a quick peace with Rome’s Germanic adversaries to return to Rome and enjoy the luxuries of imperial office.