The Roman Civil War of 68–69 AD, known as the Year of the Four Emperors, was a period of extraordinary chaos during which the loyalty and combat effectiveness of military units determined the fate of the empire. After the suicide of Nero in June 68, a rapid succession of emperors—Galba, Otho, Vitellius, and finally Vespasian—seized and lost power through armed force. The Roman army, far from being a monolithic instrument of state, fractured along regional and personal loyalties. Legions, auxiliary cohorts, and cavalry units from every frontier province marched against one another in battles that reshaped the command structure of the early empire. Understanding the composition, deployment, and allegiance of these military units is essential to grasping how a civil war of this magnitude could erupt within Rome’s highly disciplined military system. This article examines the key Roman military units that fought in the conflict, their organization, and their decisive impact on the outcome.

Roman Military Organization in the First Century AD

By the mid-first century AD, the Roman army had evolved into a professional standing force under the emperor’s direct control. The army consisted of legions (citizen heavy infantry), auxilia (non-citizen supporting troops), the Praetorian Guard, urban cohorts, and naval forces. Each legion was a self-contained formation of approximately 5,200 legionaries, organized into ten cohorts. The first cohort was double-strength (cohors milliaria) with elite soldiers, while the other nine cohorts were standard 480-man cohorts each divided into six centuries of 80 men. Each century was commanded by a centurion, with a senior centurion called the primus pilus leading the first cohort. Legionaries were equipped with the lorica segmentata or mail, the scutum shield, the gladius, and two pila. During the civil war, legions often fought with their own aquila eagle standards, which were symbols of immense pride and loyalty.

Legions: The Backbone of the Army

Legions formed the core of every major battle. Each legion was commanded by a legatus legionis, usually a senator appointed by the emperor. In civil war, legions frequently switched allegiance based on promises of pay, land grants, or personal loyalty to their commander. For example, Legio V Alaudae (“the Larks”), originally raised by Julius Caesar, fought for Vitellius at the First Battle of Bedriacum and suffered heavy losses, later being disbanded by Vespasian. Legio X Gemina, stationed in Hispania Tarraconensis, initially supported Galba but later transferred loyalty to Otho and then to Vespasian. The Danubian legions—Legio I Adiutrix, Legio II Adiutrix, and Legio XIII Gemina—played crucial roles in the rise of Otho and Vespasian. The eastern legions, including Legio III Gallica, Legio IV Scythica, and Legio XV Apollinaris, formed the backbone of Vespasian’s support. Legio VII Galbiana, raised by Galba in Spain, later became Legio VII Gemina under Vespasian. Other legions such as Legio I Germanica and Legio XV Primigenia were purged after the war. The fates of these units underscore how civil war reshaped the legionary roster.

Auxiliary Units: Cohorts and Alae

Auxiliary units provided the Roman army with light infantry and cavalry capabilities that legions lacked. Composed mainly of peregrini (non-citizens) from the provinces, auxiliaries were organized into cohortes peditatae (infantry) and alae (cavalry). There were also cohortes equitatae, mixed units of infantry and cavalry. Standard sizes were quingenaria (500 men) and milliaria (1,000 men). Infantry cohorts were often equipped with javelins, bows, or slings. During the civil war, auxiliaries were indispensable for reconnaissance, skirmishing, and securing supply lines. The Batavian cohorts, raised from the Germanic Batavi tribe, were particularly renowned for their cavalry and amphibious tactics. Initially serving Vitellius, they later defected to Vespasian under their commander Julius Civilis, who subsequently led a major revolt in 69-70 AD. Thracian, Gallic, and Spanish auxiliary units also appear in narratives of the battles. Citizenship was often granted to auxiliaries at discharge, a reward that encouraged loyalty—but during civil war, immediate promises of citizenship could sway units.

Cavalry Units: The Eyes and Hammer of the Army

Cavalry served multiple roles: reconnaissance, pursuit, flank attacks, and screening. The Roman army had two main types of cavalry: the equites legionis (legionary cavalry, about 120 per legion) and the auxiliary alae. Alae were pure cavalry formations of 500 or 1,000 horsemen, often recruited from Gaul, Thrace, or Spain. They carried lances, spatha swords, and sometimes javelins. In the civil war, cavalry engagements were frequent and decisive. At the First Battle of Bedriacum (April 69), the Vitellian cavalry under Fabius Valens outmaneuvered Otho’s forces, contributing to the Othonian defeat. Later, at the Second Battle of Bedriacum (October 69), Vespasian’s Flavian cavalry, led by the prefect Antonius Primus, broke the Vitellian lines. The pursuit after the battle was largely carried out by cavalry, resulting in heavy Vitellian casualties. Cavalry also played a key role in the street fighting in Rome, where mounted auxiliaries were used to patrol and suppress resistance.

The Praetorian Guard and Urban Cohorts

The Praetorian Guard was the emperor’s personal bodyguard and the only military unit permanently stationed in Italy. Composed of nine cohorts (initially 500 men each, later 1,000), the Guard was commanded by two Praetorian prefects. Under Nero, the Guard had been loyal, but after his fall the prefects became kingmakers. Galba disbanded the Guard and replaced it with legionaries from Spain, alienating the existing members and contributing to his murder by Otho’s supporters in January 69. Otho restored the original Guard but was later defeated by Vitellius, who executed many Guardsmen and recruited his own supporters. Vespasian, upon taking power, purged the Guard thoroughly and replaced it with veterans from his own legions, especially from the Danubian forces. The urban cohorts (cohortes urbanae) acted as a police force in Rome; during the civil war they largely remained neutral but sometimes assisted the Guard in maintaining order. Their neutrality did little to prevent the looting and violence that accompanied the fall of Vitellius.

The Fleet: Transport and Support

The Roman navy played a logistical role in the civil war. The two main fleets, the Classis Misenensis based at Misenum and the Classis Ravennas at Ravenna, provided transport for troops and supplies. Vitellius used the Ravenna fleet to move legions from the Rhine to Italy. Later, the fleet at Ravenna defected to Vespasian under the commander Lucilius Bassus, allowing Flavian forces to cross the Adriatic quickly. The Classis Misenensis also switched sides, depriving Vitellius of naval support. While no major naval battles occurred, control of the fleets ensured rapid movement of legions and prevented reinforcement of Vitellius.

The Civil War of 68–69 AD: A Military Perspective

Military units were the arbiters of power in 68–69. Each emperor’s rise and fall depended directly on the support of specific legions and auxiliary formations. The war unfolded in three main phases: the fall of Nero and the brief reign of Galba, the Otho–Vitellius conflict, and the final victory of Vespasian.

The Fall of Nero and the Rise of Galba

Nero’s loss of military support began when legions in Gaul and Spain revolted. The governor of Hispania Tarraconensis, Servius Sulpicius Galba, raised a force including Legio X Gemina and Legio VI Victrix, and declared himself emperor. The Praetorian Guard in Rome was bribed to declare for Galba, and Nero committed suicide in June 68. However, Galba alienated the legions along the German frontier by failing to pay the promised donative. When he adopted Piso Licinianus as his heir, the Rhine legions—particularly Legio I Germanica, Legio V Alaudae, Legio XV Primigenia, and Legio XIIII Gemina—proclaimed their own commander, Aulus Vitellius, as emperor. Galba’s reliance on the Spanish legions and his failure to secure the loyalty of the Rhine army set the stage for the next round of conflict. The Praetorian Guard, enraged by Galba’s disbandment, assassinated him in the Forum.

Otho vs. Vitellius: The Legions of the Rhine and Danube

After Galba was murdered by Praetorians loyal to Otho in January 69, Otho became emperor. He commanded the loyalty of the Danubian legions (I Adiutrix, II Adiutrix, XIII Gemina, and VII Galbiana) and auxiliary units from Pannonia and Moesia. Vitellius, however, had the powerful Rhine army, including Legio I Germanica, Legio V Alaudae, Legio XV Primigenia, and Legio XIIII Gemina (the latter was originally from Britain but moved to the Rhine). The two forces met near Cremona at the First Battle of Bedriacum in April 69. The Vitellian commanders, Fabius Valens and Aulus Caecina Alienus, used superior cavalry and veteran German auxiliaries to defeat Otho’s army. The Danubian legions fought bravely but were outmaneuvered; the Othonian camp was captured after fierce resistance. Otho committed suicide, and Vitellius entered Rome.

Vespasian’s Bid for Power: The Eastern Legions

Vitellius’ rule was short-lived. The governors of the eastern provinces, Titus Flavius Vespasianus (commanding the war in Judaea) and Gaius Licinius Mucianus (governor of Syria), had substantial military forces. Vespasian commanded Legio V Macedonica, Legio X Fretensis, and Legio XV Apollinaris in Judaea; Mucianus had Legio III Gallica, Legio IV Scythica, and Legio VI Ferrata in Syria—a total of six legions. Additionally, the Danubian legions that had fought for Otho switched allegiance to Vespasian. In July 69, the eastern legions declared for Vespasian. The Danubian legions under Antonius Primus (commander of Legio VII Galbiana) marched westward. At the Second Battle of Bedriacum in October 69, Primus’ Flavian forces destroyed the Vitellian army. Legio III Gallica displayed exceptional discipline, and the Flavian cavalry pursued the routed Vitellians relentlessly. The Flavian army then marched on Rome, which fell after fierce street fighting involving the Praetorian Guard and urban cohorts. During the assault, Vitellius was captured and killed.

The Siege of Rome and the Fall of Vitellius

The final phase of the civil war involved house-to-house fighting in Rome. Vitellius had mustered the Praetorian Guard, and remnants of the German legions stationed in the city fought bitterly. The Flavian forces, reinforced by troops from all over the empire, stormed the capital street by street. The Vitellian legions surrendered after Vitellius himself was dragged through the Forum and executed by the Flavian soldiers. The city was sacked by Flavian troops, though Vespasian later restored order.

Aftermath: Reorganization and Legacy

The civil war had a profound impact on the Roman military. Vespasian took immediate steps to eliminate unreliable legions and to reorganize the army. Legio I Germanica and Legio XV Primigenia were disbanded for their support of Vitellius. Legio V Alaudae was also broken up or destroyed. Other units, such as Legio XIIII Gemina, were redeployed to other frontiers. The Praetorian Guard was purged and replaced with loyal Flavian veterans, especially from Legio III Gallica and Legio VII Gemina. The auxiliary units that had defected—particularly the Batavian cohorts under Civilis—were suppressed after the Batavian revolt, though Vespasian eventually signed a treaty with the Batavi.

Military Reforms Under the Flavians

Vespasian and his successors, Titus and Domitian, strengthened the military structure. Legions were permanently stationed on frontiers (the limes) rather than being moved frequently. Fortresses were built of stone, such as the castra of Legio II Augusta at Caerleon and Legio XX Valeria Victrix at Chester. The system of limitatio was developed. Vespasian also increased the pay of legionaries and improved the status of auxiliary soldiers, granting citizenship upon honorable discharge more consistently. These reforms helped prevent large-scale civil wars for nearly a century until the Crisis of the Third Century. The new legions raised by Vespasian, such as Legio II Adiutrix Pia Fidelis and Legio IV Flavia Felix, became loyal tools of the dynasty.

Conclusion

The Roman military units that fought in the civil war of 68–69 AD were decisive in shaping the outcome. Legions such as V Alaudae, X Gemina, and XV Apollinaris; auxiliary cohorts of Batavians and Thracians; and cavalry alae from Gaul and Spain all played critical roles. Their loyalties shifted based on personal ties, pay, and political promises, demonstrating that the Roman army was not a monolithic force but a collection of powerful units with their own interests. The civil war ultimately led to the consolidation of the Roman Empire under the Flavian dynasty and highlighted the necessity of maintaining disciplined, loyal forces. For further reading, see the entries on the Year of the Four Emperors, the Battle of Bedriacum, the Legio V Alaudae, and the Praetorian Guard. These sources provide detailed accounts of the units and battles described above.