Why did the Marcomannic Wars begin in 166 CE?
A Danube crossing by 6,000 Langobardi and Obii in late 166 CE triggered the Marcomannic Wars. Repulsed by Roman detachments, the raid snowballed into a wider Marcomanni–Quadi–Sarmatian coalition just as Rome’s frontier was weakened by troop redeployments from the Parthian War and the onset of the Antonine Plague.
In late 166 CE, Cassius Dio reports that “six thousand” Langobardi and Obii crossed the Ister (Danube) into Pannonia. Roman forces under the cavalry commander Vindex and the infantry officer Candidus beat them back, and envoys approached the governor Marcus Iallius Bassus, with the Marcomannic king Ballomar (Ballomarius) mediating. What began as a large raid quickly widened as Marcomanni, Quadi, Vandals, and Sarmatian groups pressed along the middle Danube. At the same time, Rome was shifting units home from the Parthian War and facing the first wave of the Antonine Plague, creating manpower gaps that turned a single breach into a protracted frontier war.
Key Factors
A decisive border breach
The immediate spark was the crossing of the Danube by about 6,000 Langobardi and Obii into Pannonia. Roman detachments under Vindex and Candidus repulsed them, and negotiations followed through the governor Marcus Iallius Bassus, with Marcomannic king Ballomar acting as mediator.
Rapid coalition escalation
Neighboring Danubian peoples—Marcomanni, Quadi, Vandals, Sarmatians, and others—moved in the wake of the initial incursion. Embassies, shifting alliances, and pressure along the frontier turned a single raid into a multi-tribal war zone.
Roman vulnerabilities after the eastern war
The Parthian campaign had just concluded, and troops were in transit, leaving the Danube understrength. Simultaneously, the Antonine Plague began to sap manpower across the provinces, magnifying the strategic shock of the incursion.
Crisis management that revealed deeper strain
Roman authorities restricted market access to deny reconnaissance and experimented with resettling or employing some barbarians. These measures underscore how swiftly the frontier crisis widened beyond a single punitive response.
Historical Evidence
"“Six thousand Langobardi and Obii crossed the Ister [Danube],” were repulsed by Vindex and Candidus, and sent envoys to Governor Marcus Iallius Bassus; Ballomar mediated."
"After the initial fighting, embassies came seeking terms; Romans restricted markets to prevent enemy reconnaissance and resettled or employed some barbarians."
"The war was “raised by the Marcomanni together with the Quadi, Vandals, Sarmatians, Suebi, and all the barbarians in that quarter.”"
"On the plague’s spread with the returning army: “it was his fate that through the provinces by which he returned to Rome he seemed to bring a plague with him.”"
Part of Marcomannic Wars Begin
This entry anchors the ‘Marcomannic Wars Begin (166)’ event on the Roman Germanic Wars timeline by explaining the immediate trigger, coalition dynamics, and Roman vulnerabilities. For continuity, see related entries on the rain miracle (172) and Roman campaigning from Carnuntum to track how the initial breach became a long war.
More Questions About Marcomannic Wars Begin
Marcus Aurelius on the Danube: the opening campaigns, 166–168
In late 166 CE, a Langobardi–Obii raid across the Danube was crushed by Roman forces, prompting envoys led by Ballomar to sue for peace. In 168, Marcus Aurelius and Lucius Verus advanced to Aquileia, raising emergency levies amid the Antonine plague to prepare for a protracted Germanic and Marcomannic war.
Marcomanni, Quadi, and Iazyges: Rome’s opponents at the outbreak
The Marcomannic Wars opened in 166–167 with the Marcomanni under King Ballomarius (Ballomar), the Quadi, and the Sarmatian Iazyges launching Danube incursions. Allied groups like the Langobardi and Obii joined, but early crossings were repelled by Roman commanders Vindex and Candidus before Ballomarius led a peace embassy to the Pannonian governor M. Iallius Bassus.
Winter warfare on the Danube: crossings and fortified camps in 166–168
Between 166 and 168 CE, Rome fought a winter defensive war on the mid-Danube, repelling small crossings and consolidating fortified winter quarters at Carnuntum. Marcus Aurelius used Pannonia as his base, restricted frontier markets, and in 168 with Lucius Verus secured Italy and Illyricum—stabilizing the line before later river offensives.
How did the Antonine Plague set the stage for the Marcomannic Wars?
By crippling Rome’s manpower and finances just as Germanic and Sarmatian groups crossed the Danube, the Antonine Plague turned a frontier breach into a protracted war. Contemporary writers record depleted legions, delays at the capital, and emergency sales of imperial treasures—conditions that framed the Marcomannic Wars from 166.