How Were Roman Emperors Chosen?

There was no single legal procedure for choosing a Roman emperor. The office evolved without ever being defined by statute, and over the four centuries of the imperial system, emperors were made by the army, the Senate, the Praetorian Guard, hereditary succession, adoption, and at times sheer force. The result was a remarkably unstable system in which the average reign was short and violent — most emperors died in office, and many died at the hands of those who had elevated them. The full sweep of these rulers is charted in Roman Emperors.

The First Emperor and the Problem of Succession

When Augustus Caesar died in 14 CE, he left no clear mechanism for transferring power. He had adopted his stepson Tiberius, but adoption was not a constitutional process, and the Senate had to debate whether anyone could inherit the imperium at all. Tiberius was accepted because the legions in Germany and Pannonia backed him, the Senate acquiesced, and Augustus’s widow Livia used her influence to smooth the way. From this inauspicious start, every subsequent succession was improvised. The pattern that emerged — that an emperor should designate an heir, ideally one already steeped in administration — was honored more often than not, but never legally required.

The Senate’s Formal Role

In theory, the Senate confirmed each new emperor. After the death of an emperor, the Senate would ratify his imperator title, his tribunicia potestas, and his other powers, and bestow divine honors on his memory. This gave the regime a patina of legality, but the Senate was rarely the decisive actor. It could refuse a candidate the army had already acclaimed, but only at the cost of civil war. By the third century the Senate’s role had shrunk to a formality, and the real power of selection lay elsewhere. Learn more about the body itself in How Did the Roman Senate Work?.

The Power of the Army

The decisive voice in most successions was the Roman army. Legionaries were paid by the emperor, and they expected their leaders to come from their own ranks. The famous moment came in 68 CE, when Galba’s troops acclaimed him emperor while he was still governing Hispania Tarraconensis, plunging Rome into the civil war that became the Year of the Four Emperors. From Vespasian onward, the legions understood that an emperor who failed to satisfy them could be replaced by one who would. Diocletian and Constantine later tried to defuse this by dividing the empire into regional commands, so that no single army could make or unmake a ruler of the whole.

The Praetorian Guard and the Auction of Empire

Even more dangerous than the legions was the Praetorian Guard, the elite cohort of about 9,000 soldiers stationed in Rome. Because they were physically closest to the emperor, their favor was sometimes enough to decide a succession. The most notorious moment came in 193 CE, after the murder of Pertinax, when the guard literally auctioned the empire to the highest bidder. The wealthy senator Didius Julianus offered each soldier 25,000 sesterces, won the throne, and was murdered two months later. The lesson was clear: an emperor who could not pay, or who fell out of favor with the guard, would not keep his throne for long.

Heredity and the Julio-Claudians

The first dynasty, the Julio-Claudians, tried to pass power from father to son, but with uneven results. Augustus passed to Tiberius, Tiberius to his grandnephew Caligula, Caligula to his uncle Claudius, and Claudius to Nero. Each transition was marked by suspicion, murder, and crisis. The dynasty ended in 68 CE with Nero’s suicide, and a century would pass before hereditary succession became genuinely stable. The story of these rulers is told in The Julio-Claudian Dynasty.

Adoption: The Five Good Emperors

The most successful succession system was adoption, and the golden age of the system is associated with the Five Good Emperors of the second century. Nerva, Trajan, Hadrian, Antoninus Pius, and Marcus Aurelius were mostly unrelated by blood, each chosen by his predecessor for his ability. The system worked because the empire was at peace, the succession took place within the lifetime of the reigning emperor, and the candidates were seasoned administrators. It broke down with the death of Marcus Aurelius in 180 CE, when his biological son Commodus inherited a throne he was unfit to hold. Read the full story in The Five Good Emperors.

The Year of the Four Emperors

The most explicit demonstration of how emperors were chosen came in 69 CE, the Year of the Four Emperors. After Nero’s death, four generals in different provinces were acclaimed by their troops: Galba in Spain, Otho in Rome, Vitellius in Germany, and Vespasian in the east. The year saw three civil wars in quick succession. By its end, Vespasian emerged as emperor and founded the Flavian dynasty, and the lesson was plain: the empire belonged to whoever controlled the most legions. This same pattern reappeared in the third century, when the empire briefly fragmented into the so-called Crisis of the Third Century, with rival claimants ruling in different provinces.

Diocletian and the Tetrarchy

The boldest attempt to reform succession came from Diocletian, who ruled from 284 to 305 CE. Faced with a shattered empire, he created the Tetrarchy, a system of two senior Augusti and two junior Caesars, each ruling a quarter of the empire. Each Caesar was to succeed his Augustus, and the Caesars in turn were to appoint new Caesars. The system produced a smooth transfer of power in 305 CE, the first in Roman history, but collapsed almost immediately when Diocletian retired, and the empire was plunged into a new round of civil wars. The full story is in Diocletian and the Tetrarchy.

From Constantine to the End

After Constantine, the eastern and western courts developed different patterns. In the east, dynasties were more stable and rulers often handed power to sons. In the west, emperors were increasingly puppets of Germanic generals, and the last western emperor, Romulus Augustulus, was deposed in 476 CE by Odoacer, who was careful to send the imperial regalia to the eastern emperor in Constantinople. The system of imperial selection had finally exhausted itself, ending more than four centuries of improvised successions. The full arc is told in The Roman Empire: A Complete History.