| Legio | raised | P | notes | N |
|---|---|---|---|---|
| I Germanica | Republic | Y | disbanded, 70 AD | |
| I Adiatrix pia fidelis | Nero | Y | Adiutrix | Y |
| I Italica | Nero | Y | Y | |
| I Marciana | Nero | Y | disbanded, 70 AD | |
| I (Flavia) Mivervia pia fidelis | Domitian | Y | Y? | |
| I Parthica Nisibena | Severus | Y | Y | |
| I Maximiana | Tetrarchy(?) | Y | ||
| I Valentiana | Valentinians(?) | Y | ||
| I Illyricorum | Tetarachy(?) | Y | ||
| I Pontica | ? | Y | ||
| I Iovia | Diocletian? | Y | ||
| I Noricorum | ? | Y | ||
| I Flavia Gemina | ? | Y | ||
| I Flavia Mettis | ? | Y | ||
| I Flavia Constantia | ? | Y | ||
| I Flavia Gallicana Constantia | ? | Y | ||
| I Flavia Pacis | ? | Y | ||
| I Iulia Alpina | ? | Y | ||
| I Martiorum | ? | Y | ||
| I Armeniaca | ? | Y | ||
| I Isaura Sagittaria | ? | Y | ||
| II Augusta | Republic/ Augustus | Y | Y | |
| II Adiatrix pia fidelis | Nero | Y | Adiutrix | Y |
| II Italica | M. Aurelius | Y | Y | |
| II Parthica | Severus | Y | Y | |
| II Traiana fortis | Trajan | Y | Y | |
| II Flavia constantia | Tetrarchy(?) | Y | ||
| II Valentiana | Valentinians(?) | Y | ||
| II Isaura | ? | Y | ||
| II Herculia | Diocletian? | Y | ||
| II Flavia Gemina | ? | Y | ||
| II Flavia Virtutis | ? | Y | ||
| II Iulia Alpina | ? | Y | ||
| II Felix Valentis | ? | Y | ||
| II Armeniaca | ? | Y | ||
| II Britannica | ? | Y | ||
| III Augusta pia fidelis | Republic/ Augustus | Y | Y | |
| III Cyrenaica | Republic | Y | Y | |
| III Italica concors | M. Aurelius | Y | Y | |
| III Parthica | Severus | Y | ? | |
| III Diocletiana | Tetrarchy | "of the Thebaid" | Y | |
| III Gallica | ? | Y | ||
| III Isaura | ? | Y | ||
| III Herculia | ? | Y | ||
| III Iulia Alpina | ? | Y | ||
| III Flavia Salutis | ? | Y | ||
| IV Macedonica | Caesar | Y | disbanded, 70 AD | |
| IV Flavia felix | Vespasian | Y | Y | |
| IV Scythica | M. Antony? | Y | Y | |
| IV Italica | Severus? | ? | ? | |
| IV Martia | ? | Y | ||
| IV Parthica | ? | Y | ||
| IV Flavia | ? | Y | ||
| V Alaudae | Caesar | Y | destroyed under Domitian(?) | |
| V Macedonica | Republic | Y | Y | |
| V Iovia | ? | Y | ||
| VI Ferrata fidelis constans | Caesar | Y | ? | |
| VI Victrix | Republic | Y | Y | |
| VI Herculia/Herculea | ? | Y | ||
| VI Parthica | ? | Y | ||
| VII Claudia pia fidelis | Caesar | Y | Y | |
| VII Gemina | Galba | Y | Y | |
| VIII Augusta | Republic | Y | ? | |
| VIII/IX Hispana | Republic | Y | destroyed under Hadrian(?) | |
| X Fretensis | Republic | Y | Y | |
| X Gemina | Caesar | Y | Y | |
| XI Claudia pia fidelis | Republic | Y | Y | |
| XII Fulminata | Caesar | Y | Y | |
| XIII Gemina pia fidelis | Republic | Y | Y | |
| XIV Gemina Martia Victrix | Republic | Y | Y | |
| XV Apollinaris | Augustus | Y | Y | |
| XV Primigenia | Caligula | Y | disbanded, 70 AD | |
| XVI Gallica | Augustus | Y | disbanded, 70 AD | |
| XVI Flavia Firma | Vespasian | Y | Y | |
| XVII | Augustus | Y | destroyed, 9 AD | |
| XVIII | Augustus | Y | destroyed, 9 AD | |
| XIX | Augustus | Y | destroyed, 9 AD | |
| XX (Valeria) Victrix | Augustus | Y | Y? | |
| XXI Rapax | Augustus | Y | destroyed under Domitian(?) | |
| XXII Deiotariana | Augustus | Y | destroyed under Hadrian(?) | |
| XXII Primigenia pia fidelis | Caligula | Y | ? | |
| XXX Ulpia Victrix | Trajan | Y | destroyed at Amida, 359? | |
The "P" column indicates whether a Legion was ever part of the Army of the Principate, i.e. Augustus to Alexander Severus. The "N" column indicates whether a Legion is listed in the Notitia Dignitatum, the Army of the Dominate, as of the beginning of the 5th Century.
The Legions under the Tetrarchy and then for the rest of the 4th Century multiply in number but shrink in size. There are also other units, which I have not included, because they are not numbered in the traditional way. There were always, of course, the additional units of the auxilia, like cavalry and skirmishers, whose ranks originally did not need to be Roman citizens. In the Late Empire, it looks like the citizenship qualification even for Legionary troops was dropped, as recruits for the Army came to be drawn increasingly from barbarians.
Historians of the Roman Army like Goldsworthy are clearly losing interest by the time we get to the Army of the Dominate. He is content to give a diagram of the larger structure of that Army, East and West [p.204], which I have examined elsewhere. There is some logic to this, since the Legions will eventually lose their identity in these structures and be lost to history. However, if one really does want to the story of the "Complete Roman Army," one is not thereby getting it. I inevitably get the impression that the real Roman Army to someone like Goldsworthy is that of the Principate, with later details a rather annoying, if necessary, footnote. Pollard and Berry have a larger footnote but still employ a hard cut-off at 476. Heaven forbid that we should then follow the history of the Army into the age of "Byzantium," where the mobile Armies of the East are transformed into the Armies of the Themes, which we all know is a kind of phony Roman Empire. And if its history goes on for another thousand years, that is just off the radar.
The barbarization of the Roman Army, with the attendant loss of discipline and loyalty, was certainly one of the principle causes of the collapse of the Western Empire, as Arther Ferrill argues in The Fall of the Roman Empire, the Military Explanation [Thames and Hudson, London, 1986]. This was more a matter of the system being overwhelmed by numbers, however, since barbarians had been brought into the Army for a long time without harm. They had been Romanized by the experience, even as they were granted citizenship at the end of their service.
At the end of the 4th Century, however, with German refugees from the Huns pouring across the border, invited and uninvited, this was too much. There was not enough of a cadre of proper Romans to Romanize so many barbarians. Thus, while there were some spectacular defeats of Roman arms on the battlefield, as at Adrianople in 378, the most telling eventuality is that much of the Army of the Western Empire just seemed to evaporate. The Master of Horse of Gaul, who had a force of 32,500 men according to the Notitia Dignitatum, appears to have nothing at all when the Vandals, Suevi, and Alans poured across the Rhine on January 1st, 407. There was no great battle, no terrible defeat. That great force, the largest on record in the Notitia, just isn't there. This is still puzzling.
The East, of course, purged its Germans and was able to revive the discipline and loyalty of its Army with domestic recruits. Yet the Eastern Army and its successor for a thousand years would never turn away barbarian recruits. One of its most famous formations, the Varangian Guard, would be entirely foreign mercenaries, which was valuable for its personal attachment to the Emperor and its immunity to domestic political involvements.
The weakness of the Army of the 4th century, however, does reveal something important about Roman history. The Army of Augustus was a professional force, not a levy of the Citizens as the Roman Army had been originally. When Hannibal slaughtered whole Legions, barbarian recruits were unnecessary, and at the time unthinkable, because Rome had enough Citizens to make good the losses. This did not happen when an oversupply of barbarians began to denature the Army and Germanize the state in the West. While the later Mediaeval Army was always professional, and was never a mass levy, the formation of the Themes did mean that soldiers were both professionals and Citizens. This worked well for several centuries, until the Themes were neglected and the Emperors came to rely too much on purely professional and mercenary forces. For anyone looking for cycles in history, this is a good one. The Themes were neglected and withered away just as had the Limitanei. In each case, what looks like a complex and sophisticated system just seems to suddenly vanish. This did not produce quite the same effect as in the 4th Century, since the Tagmata did not become a Trojan Horse for something like Germanic infiltration; but it did mean that the Army was simply insufficient for the defense of Romania in the 11th Century, and after a disaster like Manzikert (1071) the Empire simply did not have the depth of organization to make good its losses. Unlike Zeno, Alexius Comnenus could not draw on some untapped reserve of native manpower. The advance of the Turks, initially disorganized and even unauthorized, would only be checked by the First Crusade.
By then, of course, the old Legions were a distant memory. But we should keep in mind the lesson of their origin. As, I think, Goethe said of the French Revolutionary Army, the mass of the early Roman Army was such as to sweep away "the junk of history." The Western Army, but not the Eastern, was similarly swept away by Germans, as later the top-heavy Tagmata were swept away at Manzikert.
The distribution of the Armies of the Principate, and other details about the Legions, are available through the links below.
The Augustan Roman Army in 14/24 AD
The Flavian Roman Army in 74/79 AD
The Antonine Roman Army in 138/150 AD
The Severan Roman Army in 230 AD
The Theodosian Roman Army, c.408 AD
The Organization of the Themes and Exarchates, 668 AD
The Organization of the Themes and Tagmata, 840 AD
Philosophy of History, Military History