Marcus Aurelius

Unlike Antoninus Pius, Marcus Aurelius and Lucius Verus had to face continuous threats to the stability of the empire. In 162 the Parthians invaded the eastern Roman provinces of Syria and Armenia; Lucius Verus led the Roman armies to the counter-attack and in 164 the Romans occupied again Ctesiphon, the enemy capital and briefly restored their rule over Mesopotamia; the victory however brought an unexpected and bitter fruit: a pestilence (now thought to have been a bubonic plague) developed among the Roman legionaries and they brought it back with them at the end of the campaign; in the following years the pestilence spread to the whole empire; a famine followed the disease and further weakened the economy and reduced the population.

The two emperors had just repelled the Parthians in the east, when a new threat arose in the west; the Marcomanni, a German tribe living in today's Bohemia, in association with other tribes (Quadi, Vandals and Sarmatians) attacked the Roman colonies along the Danube in Germany, Austria and Hungary. They were most likely forced to do this because they were in turn attacked by other tribes coming from Central Asia.

After the death of Lucius Verus in 169, Marcus Aurelius had to bear the full responsibility of defending the empire. Being a man of great culture, he would have preferred to have time for his philosophic studies, but he felt it was his duty to personally lead the many campaigns caused by the increasing pressure on the northern border of the empire.

He died in 180 in Vindobona (today's Vienna) where he was closely following the preparation of yet another campaign against the Marcomanni. Many historians set at his death the beginning of the decadence of the Roman Empire as some of its key causes made their appearance during the reign of Marcus Aurelius: a weakened economy, a substantial reduction in population and the first barbarian invasions, i.e. the irruption in Europe of nomadic peoples from Central Asia.