The Year of the Four Emperors & the Demise of Four Roman Legions
During the Year of the Four Emperors (69 CE), the fight between Vitellius and Vespasian would ultimately bring about the demise of four legions, the XV Primigenia, I Germanica, IIII Macedonica, and XVI Gallia. All four of these legions had previously served the Roman Empire with distinction under such leaders as Pompey and Octavian but made what turned out to be the wrong choice in 69 CE. While there remains a question concerning the loyalty of XV Primigenia, three of the legions made the mistake of supporting Vitellius.
Roman Expansion
Originally, the Roman army consisted of a citizen-based militia recruited from the propertied citizens. However, the consulship of Gaius Marius (l. 157-86 BCE) brought a number of changes. Property ownership not being a requirement anymore, the Marian Reforms allowed the Roman army to reinvent itself as a professional fighting force. Another significant change came in the reorganization of the legion. The new legion was broken into centuries and cohorts. A centurion, commanded a century of 80 men (not 100) – six of these centuries equaled a cohort of 480 men. With the rebirth of the legion, the legionary became a well-trained and disciplined foot soldier, fighting as part of a well-organized unit.
With Gnaeus Pompey’s (106-48 BCE) venture into Spain and Caesar’s assault on Gaul, the number of legions increased dramatically. As the empire expanded, more legions were necessary to keep the frontier borders secure. Before the time of the first Roman emperor Augustus (Octavian) (27 BCE - 14 CE), the Roman army was constantly on the march, relying on temporary camps more than permanent fortresses. As the borders of the empire expanded, permanent fortresses began to replace the marching camps. This move helped to stabilize the frontier.
After his successful return to Rome, Augustus wanted to be assured that his legionaries were loyal to him and not a usurper. Each soldier had to take an oath of allegiance to the Roman emperor, the ius iurandum: an oath renewed every year, on 3 January. Next, Augustus reduced the number of legions from 60 to 28. These 28 legions became 25 after the Roman commander Publius Quinctilius Varus (46 BCE - 9 CE) lost three legions - XVII, XVIII, and XIX - in the disastrous Battle of Teutoburg Forest. Most of the 25 legions were stationed in the troubled provinces and along the borders - only nine cohorts were assigned to Italy with three of these in Rome. In the end, Rome had a standing army of 150,000 legionaries and 180,000 auxiliary infantry and cavalry.
The empire’s expansion brought them into contact with a population of different customs, languages, and religions. To deal with this disparity and maintain the peace, the Pax Romana, the Romans relied on the army. According to historian Stephen Dando-Collins in his book Legions of Rome, the 1st and early 2nd century CE was the golden age of the legion when they "swept all before them." He considered the Roman legion of the imperial era to be "a triumph of organization.” He added, "… each component from heavy infantry to cavalry, artillery to supporting auxiliary light infantry, fitting neatly together to form a solid, self-contained military machine." (10)
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