List of Roman legions

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This is a list of Roman legions, including key facts about each legion, primarily focusing on the Principate (early Empire, 27 BC – 284 AD) legions, for which there exists substantial literary, epigraphic and archaeological evidence.

When Augustus became sole ruler in 31 BC, he disbanded about half of the over 50 legions then in existence. The remaining 28 legions became the core of the early Imperial army of the Principate (27 BC – AD 284), most lasting over three centuries. Augustus and his immediate successors transformed legions into permanent units, staffed by entirely career soldiers on standard 25-year terms.

During the Dominate period (near the end of the Empire, 284–476), legions were also professional, but are little understood due to scarcity of evidence compared to the Principate. What is clear is that late legions were radically different in size, structure, and tactical role from their predecessors, despite several retaining early period names. This was the result of the military reforms of Emperors Diocletian and Constantine I, and of further developments during the 4th century.

The legions were identified by Roman numerals, though the spelling sometimes differed from the modern standard. For example, in addition to the spellings "IV", "IX", "XIV", "XVIII" and "XIX", the respective spellings "IIII", "VIIII", "XIIII", "XIIX" and "XVIIII" were commonly used.^[1] Legions also bore a cognomen or nickname. While neither a legion's number or cognomen were likely unique enough to identify it, the combination of the two is usually needed to identify a specific legion. For example, both Legio III Cyrenaica and Legio III Gallica were distinct, long-standing legions of the late Republic and Imperial periods. To visually identify legions, they also bore a specific emblem, a symbolic representation of the legion, frequently an animal or mythological figure, which appeared on the vexillum, a small rectangular flag that served as the legion's standard and carried both the emblem and name of the legion.