Roman Republic and Roman Empire
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| Varied over unit lifetime. Approx. 3,500 fighting men + support at the time of creation. Expanded and given the cognomenGemina in 31 BC.
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| Babylon in Egypt (1st century) Roman Dacia (89 - c. 270) Dacia Aureliana (270 - ?? ) Babylon? (c.400)
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| Gemina, "The twin" (since 31 BC)
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| Julius Caesar, Augustus, Roman Emperors
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| Lion
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! style="text-align: left;" | Notable battles or wars:
| Gallic Wars (58-51 BC) Battle against the Nervians (57 BC) Battle of Gergovia (52 BC) Battle of Alesia (52 BC) - uncertain Battle of Dyrrhachium (48 BC) Battle of Pharsalus (48 BC) Battle of Thapsus (46 BC) Battle of Munda (45 BC) Battle of Actium (31 BC) 1st and 2nd Battles of Bedriacum (69) Vexillationes of the 13th participated in many other campaigns.
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| Julius Caesar, Marcus Salvius Otho, Marcus Antonius Primus
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Legio XIII, the 13th Legion (after 31 BC known as Legio XIII Gemina, the twin legion), is one of the more historically noteworthy Roman legions, as it was one of Julius Caesar's armies used in Gaul, and also for his civil war. It was the legion he famously crossed the Rubicon with on January 10, 49 BC. After Caesar's victory, the legion was retired and its veterans settled, in 45 BC
After 134 years as an active legion, XIII Gemina was made a garrison army in the province of Dacia (modern Romania) by Domitian, in 89, although vexillationes from the legion were still used in active campaigns. The legion was relocated when the Dacia province was evacuated, and restationed in Dacia Aureliana.
It is unknown when the legion was finally disbanded, but it is mentioned in Roman records as late as the early 5th century; The Notitia Dignitatum (c.400) mentions a "13th twin legion", although it has it stationed in Babylon.
After the end of the Gallic wars, the Roman senate refused Caesar his second consulship, ordered him to give up his commands, and demanded he return to Rome to face prosecution. Forced to choose either the end of his political career, or civil war, Caesar brought Legio XIII across the Rubicon river and into Italy. The legion remained faithful to Caesar during the resulting civil war between Caesar and the conservative Optimates faction of the senate, whose legions were commanded by Pompey. Legio XIII was active throughout the entire war, fighting at Dyrrhachium (48 BC) and Pharsalus (48 BC). After the decisive victory over Pompey at Pharsalus, the legion was to be disbanded, and the legionaries "pentioned off" with the traditional land grants. However, the legion was recalled for the Battle of Thapsus (46 BC) and the final Battle of Munda (45 BC). After Munda, Caesar disbanded the legion, retired his veterans, and gave them farmlands in Italy.
A fictionalized account some of the actions of Legio XIII Gemina during the struggle between Julius Caesar and the Optimates faction under Pompey can be seen in the joint HBO/BBC/RAI television production Rome.
