The frontier legions as a political force

Ultimately the praetorian guards’ political power was overshadowed by that of the major groupings of frontier legions. Tacitus famously stated that the events of 68-9 CE meant that ‘‘a well-hidden secret of the principate had been revealed. It was possible, it seemed, for an emperor to be chosen outside Rome’’ (Hist. 1.4, tr.

Wellesley). Galba initially had the backing of troops from Spain, but lost military support allegedly because of his stinginess and authoritarian character (Dio 64.3.3, 4.1; Suet. Gal. 16). Subsequently Otho was backed, unsuccessfully, by the praetorians. Vitellius owed his brief rule to the backing of the legions of the Rhine frontier (Suet. Vit. 8.2), and Vespasian his much longer grip on power to support from the eastern legions who proclaimed him emperor and, crucially, the legions of the Balkan provinces under Antonius Primus who actually defeated Vitellius’ army for him (Tac. Hist. 2.85-6). The fourth-century writer Aurelius Victor, writing with nearly three centuries of hindsight of similar civil wars, claimed:

[Vespasian], because envoys from the Moesian and Pannonian armies had arrived.... seized the imperial power. For the soldiers mentioned above, after they had discovered that Otho had been made emperor by the praetorians and Vitellius by the German legions, in rivalry, as is their custom, so they should not seem different, urged on Vespasian, upon whom the Syrian cohorts had already agreed. (Caes. 8.2-3, tr. Bird)

This is an interesting reflection on the spirit of competition between the different regional army groupings. As the first emperor from outside the Julio-Claudian aristocracy of Rome, Vespasian only won power by the sword, and his military ability and popularity with elements of the army were the only real qualifications he had to be princeps.

Through much of the second century ce, the adoption of intended successors ensured that imperial accessions were largely peaceful (Peachin, this volume). However, Trajan’s support from the German legions was important in ensuring his adoption by Nerva and his subsequent accession, as they provided an important counter to the power of the praetorians who opposed Nerva. Indeed, one ancient writer (the author of the fourth-century Epitome on the Emperors) hints that Trajan’s succession may have been more of a military coup than the orderly adoption depicted in the official version (Epitome 12.8-9). The aftermath of the death of Commodus saw a situation not unlike that of 68-9 ce, with Pescennius Niger supported by the eastern legions, Clodius Albinus’ support in the garrison of Britain, and Septimius Severus drawing on the Danubian legions to achieve ultimate power against the praetorians in Rome as well as his two rivals (see Dio 73.14.3 for the contenders and their support).