According to Cassius Dio, legions marked with an asterisk were still in existence in A.D. 233.
1st Legion: Founded by Pompey the Great in 84 B.C. Briefly held title “Augusta,” bestowed by the emperor Augustus c. 25 B.C. for Cantabrian War service, taken away by Marcus Agrippa in 19 B.C. for cowardice. In early first century had the unofficial title “Germanica” for service in German campaigns, but this soon fell into disuse. Believed to have been abolished by the emperor Vespasian at the time of its A.D. 74 discharge.
2nd Augusta Legion: Founded by Pompey the Great in 84 B.C. Granted “Augusta” title, meaning “Augustus’s,” by the emperor Augustus, c. 25 B.C., for Cantabrian War service.*
3rd Augusta Legion: Founded by Pompey the Great in 65 B.C. Granted “Augusta” title by the emperor Augustus c. 25 B.C.*
3rd Cyrenaica Legion: Origin unclear. Served under both Caesar and Lepidus in 40s B.C. Its title suggests that it was raised in the province of Cyrenaica.*
3rd Gallica Legion: Originally one of Pompey’s legions, surrendered to Caesar in Spain; discharged in Transalpine Gaul in 48 B.C. and reenlisted there for Caesar, with a title apparently designed to differentiate it from the existing 3rd Legions.*
4th Macedonica Legion, later 4th Flavia Legion: Founded by Pompey the Great in 65 B.C. Could have taken title “Macedonica” in 33 B.C. when its recruitment was transferred from Spain to Macedonia. Another theory is that it took “Macedonica” title for its performance at 42 B.C. First Battle of Philippi, where it was almost wiped out. Renamed 4th Flavia by the emperor Vespasian in A.D. 70, reason unknown.*
4th Scythica Legion: In Mark Antony’s army in Macedonia in 31-30 B.C. Raised in the East by him or by Brutus and Cassius. Taken into Octavian’s forces 31-30 B.C. Origin of Scythica title unknown.*
5th Alaudae Legion: Founded by Caesar in Transalpine Gaul in 52-51 B.C. Title is from Celtic for “Crested Larks.” Caesar apparently gave it the number 5 after discharging Pompey’s 5th in 49 B.C. Wiped out prior to A.D. 93, in Domitian’s reign. Not reenlisted.
5th Legion, later 5th Macedonica Legion: Founded by Pompey the Great in 65 B.C. Reenlisted by Caesar in Spain in 47 B.C. to replace the legion’s Pompeian enlistment he disbanded in 49 B.C. “Macedonica” title may stem from the Battle of Philippi.*
6th Ferrata Legion: Founded by Pompey the Great in 65 B.C. Loaned to Caesar in 52-51 B.C., then went back to Pompey’s forces in Spain. Part that went over to Caesar after Pharsalus in 48 B.C. adopted “Ferrata” title, meaning “Ironclad.”
6th Victrix Legion: Part of above legion that was split at Pharsalus. This component continued to fight for Pompey until it surrendered to Caesar at Thapsus in 46 B.C., after which he reenlisted it as his own. “Victrix,” meaning “Conqueror,” title apparently granted c. A.D. 9 by the emperor Augustus for Pannonian War service.*
7th Claudia Pia Fidelis Legion: Founded by Pompey the Great in 65 B.C. Granted “Claudia Pia Fidelis” title, meaning “Claudius’s Loyal and Patriotic,” following the Scribonianus Revolt, by the emperor Claudius in A.D. 42. The “Pia Fidelis” fell into disuse.*
8th Augusta Legion: Founded by Pompey the Great in 65 B.C. Granted “Augusta” title by the emperor Augustus, c. 25 B.C., apparently for Cantabrian War service.*
9th Hispana Legion: Founded by Pompey the Great in 65 B.C. “Hispana” title, meaning “Spain’s,” awarded for service in the Cantabrian War. Ceased to exist in reign of the emperor Hadrian, apparently wiped out, c. A.D. 122 in Britain.
10th Legion: Founded by Julius Caesar in 61 B.C. Had unofficial title “Fretensis,” said to mean “of the Strait of Messina” and relating to Octavian’s 38-36 B.C. Sicilian battles with Sextus Pompey.*
10th Gemina Legion: Created in 31 B.C. combination by Octavian of a non-Italian 10th Legion of Antony with an unidentified non-Italian legion. Title means “Twin.”*
11th Claudia Pia Fidelis Legion: Founded by Julius Caesar in 58 B.C. Granted “Claudia Pia Fidelis” title along with the 7th by the emperor Claudius in A.D. 42. The “Pia Fidelis” fell into disuse.*
12th Fulminata Legion: Founded by Julius Caesar in 58 B.C. According to Dio, it was granted “Fulminata” title, meaning “Thunderbolt,” by the emperor Marcus Aurelius in A.D. 174 for a victory in a thunderstorm against Germanic tribes, but its men were using the title in the late republic, and its coins bore the thunderbolt logo in the first century.*
13th Gemina Legion: Founded by Julius Caesar in late 58 B.C. Gained “Gemina” title, meaning “Twin,” by the emperor Augustus in 31-30 B.C., when combined with an unidentified legion.*
14th Gemina Martia Victrix Legion: Founded by Julius Caesar in late 58 B.C. Believed to have gained the “Gemina Martia” title in combination with the Martia Legion in 31-30 B.C., and granted the additional “Victrix” title by Augustus c. A.D. 9 for service in the Pannonian War. The “Martia” and “Victrix” had fallen into disuse by the middle of the first century.
15th Legion: Founded by Julius Caesar in 54 B.C. For a time took the title “Apollinaris,” meaning “of [the god] Apollo.”*
16th Legion, later called the 16th Gallica Legion, and later still the 16th Flavia Legion: Founded by Julius Caesar in 54 B.C. in Gaul. Granted “Flavia” title by the emperor Vespasian in A.D. 70, reason unknown.*
17th Legion: Founded by Julius Caesar in 49 B.C. Believed wiped out in Africa in that year. Reformed in 42 B.C. by Antony, and then was one of Varus’s three legions lost in the A.D. 9 Teutoburg disaster. Never re-formed.
18th Legion: As above, except the legion was re-formed, apparently by Nero in A.D. 66, for his planned Egyptian operation. Four cohorts then served in the Judean offensives of A.D. 67-70, and six on the Rhine during the Civilis Revolt. Believed to have been combined by Vespasian in A.D. 70 with the 7th Galbiana to create the 7th Gemina.
19th Legion: Founded by Julius Caesar in 49 B.C. Wiped out by Germans under Arminius (Hermann) in Teutoburg Forest, A.D. 9. Never re-formed.
20th Valeria Victrix Legion: Started as two legions. The 20th was founded by Julius Caesar in 49 B.C. The Valeria Legion, meaning “Powerful,” recruited by Pompey in 84 B.C., was given the vacant 20th number by the emperor Augustus in 31-30 B.C. Augustus apparently granted it the “Victrix” title c. A.D. 9 for Pannonian War service.*
21st Rapax Legion: The 21st was founded by Julius Caesar in 49 B.C. Indications are that the Rapax Legion, meaning “Rapacious,” was founded in Spain for Pompey the same year, and originally called the Indigena, the “Native” or “Home-Grown” Legion. Given the vacant 21st number by the emperor Augustus in 31-30 B.C. One of two legions wiped out by the Dacians and Germans prior to A.D. 93 during the reign of Domitian. Never re-formed.
22nd Deiotariana Legion: The Deiotariana Legion was formed by Julius Caesar in 47 B.C., from remnants of two legions of King Deiotarus of Galatia. It was given the vacant number 22 by Augustus in 30-31 B.C. Ceased to exist during the reign of Hadrian; said to have been wiped out in the East in A.D. 135.
(In order of creation)
15th Primigeneia Legion: Raised by the emperor Caligula in A.D. 39, apparently for his aborted British campaign. The title said to have been bestowed by the next emperor, Claudius, seemingly in relation to the goddess Fortune, a leading military deity. No reason known for use of the number 15. Ceased to exist in the reign of Vespasian; apparently abolished in A.D. 79 because of its poor record during the Civilis Revolt.
22nd Primigeneia Legion: Same origin of legion and of name as for the 15th Primigeneia. No known reason for use of the number 22.*
1st Italica Legion: Founded by the emperor Nero in A.D. 66, for his planned but never executed invasion of Parthia. Originally equipped as a Greek-style spear phalanx. Took its name as the first legion recruited in Italy south of the River Po in almost 120 years.*
1st Classica Legion (Legion of the Fleet): Recruited by Nero from sailors of the Roman navy in A.D. 68. Saw civil war action in A.D. 69. Abolished by Vespasian in A.D. 70.
1st Adiutrix Legion: Raised by the city of Vienne in Gaul for the emperor Galba in A.D. 68, as a supporter of, or assistant to, the 1st Italica Legion, then stationed nearby at Lyons.*
7th Gemina Legion: Originally a second 7th Legion, founded by Galba in the Spanish recruiting grounds of the original 7th in A.D. 68 and called for a time the 7th Galbiana, “Galba’s 7th.” Apparently combined by Vespasian with the 18th Legion in A.D. 70; as a result, took the “Gemina” title.*
2nd Adiutrix Legion: Raised by the city of Vienne for the emperor Vitellius in A.D. 69, as a supporter of, or assistant to, the 2nd Augusta Legion, which had its recruiting grounds in the same province. Even adopted the 2nd Augusta’s emblem, the flying horse Pegasus.*
2nd Classica Legion (Legion of the Fleet): Recruited by the emperor Vitellius from sailors of the Roman navy in A.D. 69. Surrendered without a fight to forces of Vespasian, who abolished it in A.D. 70.
1st Minervia Legion: Founded by the emperor Domitian c. A.D. 82. Named after his patron deity, the war goddess Minerva.*
2nd Traiana Legion: Raised by the emperor Trajan in A.D. 100-102 to release an existing legion for his Dacian Wars, and named after him.*
30th Ulpia Legion: Raised by the emperor Trajan in A.D. 100-102 for service in his Dacian Wars, and given the number 30 because it would be Trajan’s 30th legionary unit, together with his family name.*
2nd Italica Legion: Raised in Italy south of the Po River by the emperor Marcus Aurelius c. A.D. 165, for his Germanic Wars.*
3rd Italica Legion: Same as for the 2nd Italica Legion.*
1st Parthicae Legion: Founded by the emperor Septimius Severus c. A.D. 197 for eastern operations, apparently gaining its title for defeating Parthians invading Mesopotamia.*
2nd Parthicae Legion: Same as for the 1st Parthicae Legion.*
3rd Parthicae Legion: Same as for the 1st Parthicae Legion.*