Marcus Aurelius and Lucius Verus became emperors thanks to Hadrian, who asked Antoninus Pius to make them emperor after Antoninus's death as a prerequisite for obtaining the throne. Marcus Aurelius and Lucius Verus were adopted sons. While, the reign of Anoninus Pius was uneventful, the reign of Marcus Aurelius and Lucius Verus was one of continuous warfare.
Vologases IV of the Parthian Empire invaded Armenia in 161, to install a pro-Parthian king. The governors of Cappadocia and Syria were defeated by the Parthians, so the Romans had to send a large army eastwards. Marcus Aurelius remained in Rome as the senior emperor and Lucius Verus commanded the Roman Army in the east. In 163, the Romans reconquered Armenia, but Osroene was conquered by the Parthians.
The Roman army in the frontier regions were depleted, leading to raids of the Chatti across the Taunus hills into the Agri Decumates in 162, who were repulsed in the same year.
Due to the threat of the Picts in Britain, the Romans retreated from Antonine Wall to Hadrian's Wall in 163/164.
The Romans invaded Mesopotamia in 163, conquering Osroene, Nisibis, Seleucia and Ctesiphon in 165, and advancing into Media in 166.
In 166 or 167, some Langobardi, Ubii and Lacringi invaded Pannonia, who could be convinced to make peace.
Peace was made with the Parthian Empire in 167, and a part of Northern Mesopotamia with Nisibis was ceded to the Roman Empire.
The Vandals, Iazyges and Costoboci invaded Dacia in 167, killing the Roman governor. A counter offensive versus the invaders of Pannonia and Dacia came relatively late in 168, because of the outbrake of the Antonine Plague, killing millions of Romans. Marcus Aurelius and Lucius Verus headed the Roman army themselves. That year, the Marcomanni and the Victuali invaded Pannonia, who quickly retreated when the Roman army arrived.
Lucius Verus died in 169, making Marcus Aurelius sole emperor of the Roman Empire.
In 169-170, the Romans had a hard time subduing the Iazyges during an invasion, when, in 170, the Costoboci invaded the Roman Empire, ravaging Thrace and destroying the temple of the Eleusinian Mysteries near Athens and a coalition of Germanic tribes under king Ballomer of the Marcomanni ravaged Noricum and started besieging the city of Aquileia in Italy, defeating a Roman army that tried to relieve the city. The Chauci invaded Dacia, and were defeated by a coalition of Romans and free Dacians.
Marcus Aurelius had to evacuate the lands of the Iazyges to fight the Marcomanni, who were ousted from Roman territory in 171. In 171, he made treaties with the Quadi, Iazyges, Hasdingi Vandals and Lacringi in order to be able to fight the Marcomanni. In 172, the Marcomanni, Varisti and Cotini were subdued by the Romans.
In the meanwhile, a revolt of the Bucoli in Egypt was supressed, by the hero of the Parthian wars, general Avidius Cassius in 172.
In 173, the Quadi were defeated, who had aided the Marcomanni in 172, despite the treaty. Now, Germanic tribes on the Rhine borders started to invade Gaul. General Didius Julianus repulsed invasions of the Chatti and the Hermunduri across the Rhine and of the Chauci along the coast of Gallia Belgica. Another invasion of Quadi lands lead to the definite subdual of this tribe.
In 175, Marcus Aurelius defeated the Iazyges. He planned to create two new provinces, Marcomannia and Sarmatia, named after the Sarmatian Iazyges. But this planned was stalled by the proclamation as emperor by Cassius in Syria. Cassius heard the wrong news that Marcus Aurelius had died, and immediately claimed the throne. When it became apparent that Marcus Aurelius was still alive and marching towards Syria and that most Roman armies still supported him, including the Roman soldiers in nearby Cappadocia, Cassius was killed by one of his soldiers.
In 177 the Quadi rebelled, followed by the Marcomanni. Marcus Aurelius campaigned against them, and against the Cotini, who were resttled into the Roman Empire. Victory was near for the Romans, when he died in 180.