Legio XIII Gemina
Sestertius minted in 248 by Philip the Arab to celebrate Dacia province and its legions, V Macedonica and XIII Gemina. Note the eagle and the lion, V's and XIII's symbols, in the reverse.
Active	57 BC to sometime in the 5th century
Country	Roman Republic and Roman Empire
Type	Roman legion (Marian)
Role	Infantry assault (some cavalry support)
Size	Varied over unit lifetime. Approx. 3,500 fighting men + support at the time of creation. Expanded and given the cognomen Gemina in 31 BC.
Garrison/HQ	Burnum, Illyricum (1st century BC) Emona, Pannonia (1st century) Augusta Vindelica, Germania Superior Poetovio, Pannonia (1st century) Roman Dacia (89 - c. 270) Dacia Aureliana (since 270) Babylon in Egypt (400s)
Nicknames	Gemina, "The twin" (since 31 BC)
Mascot	Lion
Engagements	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.
Commanders
Notable commanders	Julius Caesar, Marcus Salvius Otho, Marcus Antonius Primus

Legio XIII, known as Legio XIII Gemina ("twin legion") after 31 BC, 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. The legion was in Vienna in the 5th century. The symbol of the legion was the lion.

Legio XIII was levied by Julius Caesar in 57 BC, before marching against the Belgae, in one of his early interventions in intra-Gallic conflicts.

During the Gallic wars (58-51 BC), Legio XIII was present at the Battle against the Nervians, the siege of Gergovia, and while not specifically mentioned in the sources, it is not unreasonable to assume that Legio XIII was also present for the Battle of Alesia.

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.

Augustus reconstituted the legion once again in 41 BC to deal with rebellion of Sextus Pompeius (son of Pompey) in Sicily.

Legio XIII was reinforced with veteran legionaries from other legions, and acquired the cognomen Gemina ("twin"), after the war against Mark Antony and the Battle of Actium. Augustus then sent the legion to Burnum (modern Knin), in Illyricum, a Roman province in the Adriatic Sea.

In 16 BC, the legion was transferred to Emona (now Ljubljana) in Pannonia, where they dealt with local rebellions.

After the disaster of the Battle of the Teutoburg Forest in 9, the legion was sent as reinforcements to Augusta Vindelica (Augsburg), Germania Superior, to prevent further attacks from the Germanic tribes.

Emperor Claudius sent them back to Pannonia around 45; the legion camped at Poetovio (modern Ptuj, Slovenia).

In the year of the four emperors (69), XIII Gemina supported first Otho and then Vitellius, two of the defeated emperors, fighting in the two Battles of Bedriacum.

In 89, Domitian transferred the legion from their camp in Vienna to Dacia (in Apulum, modern Alba Iulia, Romania), to garrison the province. The legion was relocated when the Dacia province was evacuated, and restationed in Dacia Aureliana.

In the 5th century, according to Notitia Dignitatum, a legio tertiadecima gemina was in Babylon in Egypt, a strategic fortress on the Nile at the traditional border between Lower Egypt and Middle Egypt, under the command of the Comes limitis Aegypti.

• 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.

• Roman legion
• List of Roman legions

• livius.org account
• LEG XIII GEM, Austrian re-enactment group
• Legio XIIII Gemina Martia Victrix (Roman Military Research Society)