Legion: chronological

Legion: infantry unit in the Roman army.

Julius Caesar

Before Caesar:

VII, later known as Claudia Pia Fidelis
VIII, later known as Augusta
VIIII, later known as Hispana (disappears 161?)
X Equestris, later known as Gemina

Julius Caesar, 58 BCE

XI, later known as Claudia Pia Fidelis
XII, later known as Fulminata

Caesar, 57 BCE:

XIII, later known as Gemina
XIV, later known as Gemina (destroyed 54 by Ambiorix and immediately reconstituted)

Caesar, 53 BCE:

XIV (reconstituted)
XV (destroyed 49-48 in Africa)
XVI (destroyed 49-48 in Africa)

Caesar, 52 BCE:

V Alaudae (disbanded 70)
VI Ferrata

Caesar, 49-48 BCE (using consular numbers):

I, later known as Germanica (disbanded 70)
II
III, later known as Gallica
IIII, later known as Macedonica (reconstituted 70 as Flavia)

Timeline of legions

Pansa, 43 BCE:

I
II, later known as Augusta
III, later known as Augusta
IIII
V

Mark Antony:

Portrait of Augustus as pontifex maximus

Octavian, 41-40 BCE:

V Macedonica
VI Victrix
X Fretensis
XV Apollinaris
XVI Gallica (reconstituted 70 as Flavia)
XVII (lost 9 CE)
XVIII (lost 9 CE)
XIX (lost 9 CE)

Octavian 30 BCE:

Battle in Teutoburg Forest:

XVII, XVIII, XIX destroyed

Caligula, 39 CE:

XV Primigenia (disbanded 70)
XXII Primigenia

Nero

Nero, 66:

I Italica

Nero, 68:

I Adiutrix

Galba, 68:

VII Galbiana: later known as VII Gemina (after merging with I Germanica)
Brief existence of I Macriana Liberatrix

Vespasian, 70:

II Adiutrix
I Germanica, V Alaudae and XV Primigenia disbanded
IIII Flavia Felix (formerly IIII Macedonica)
XVI Flavia Firma (formerly XVI Gallica)

Domitian

Domitian, 83:

I Minervia

War against Decebalus, 92:

XXI Rapax destroyed

Trajan, 105:

Uncertain date:

VIIII Hispana destroyed (by the Parthians in 161?)
XXII Deiotariana destroyed (by Bar Kochba?)

Marcus Aurelius, 168:

Septimius Severus

Septimius Severus, 197:

Heliogabalus, 220:

Disbanded: III Gallica

Severus Alexander, 230?

IIII Italica

Gordian III, 238?:

Disbanded: III Augusta
Reconstituted: III Gallica

Philip the Arab (244-249)?

VI Hispana

Valerian, 253:

Reconstituted: III Augusta

Laelianus (268-269)?

VI Gallicana

The following units are known from late-ancient sources and are called "legions". In fact, however, they were no longer the elite troops of the Roman empire. The main forces were cavalry units; the legions were the garrisons of the frontier zone.

Probus

Aurelian (270-275):

Probus (276-282)

Before Diocletian:

Diocletian

Diocletian (284-305):

Constantius I Chlorus (293-306):

Re-enactment of a Late Roman legionary

I Flavia Gallicana Constantia
I Flavia Martis
XII Victrix

Constantius II (337-361)

Constans (337-350):