Cohors I Aelia Dacorum

Cohors prima Aelia Dacorum ("1st Aelian Cohort of Dacians") was an infantry regiment of the Auxilia corps of the Imperial Roman army. It was first raised by the Roman emperor Hadrian (r. AD 117-38) in the Roman province of Dacia not later than AD 125 and its last surviving record dates c. 400. It was deployed, for virtually its entire history, in forts on Hadrian's Wall on the northern frontier of Britannia province.

Foundation
The regiment carries the epithet Aelia, implying that it was either founded, or honoured for valour, by the emperor Hadrian (Publius Aelius Hadrianus, ruled AD 117-38). It was founded between AD 117 and c. 125, the date of its first datable attestation. The Dacorum name implies that its initial recruits were mainly ethnic Dacians from the recently conquered Roman province of Dacia (annexed in 106 by the emperor Trajan). According to Holder, the regiment was transferred to Britain immediately after it was recruited, in accordance with a general imperial policy of deploying auxiliary units far from recently conquered (or pacified) regions in order to assure their loyalty (e.g. 7 British regiments, raised during the Flavian period (71-96), are attested as deployed in Dacia in the reign of Hadrian).

Structure
The regiment's earliest datable inscription (c. 125) attests a Cohors I Dacorum, without the Aelia title. This may imply that the title was not accorded before 127, its first attestation. It has been suggested that Cohors I Dacorum was originally a quingenaria regiment (480-strong, divided into 6 centuriae of 80 men each). This is supported by the fact that the earliest attested commander, Marcus Cludius Severus, held the rank of praefectus. In contrast, all of Severus' known successors were styled tribunus, normally an indication of milliaria size (800-strong, 10 centuriae). It has been suggested that the regiment acquired its Aelia title when it was upgraded to milliaria status in 127. There is no evidence that the unit ever acquired a cavalry contingent: in no inscription is the regiment ascribed the epithet "EQ" (for equitata = "equipped with cavalry"), and no cavalry officer (decurion) or trooper (eques) is attested.

Garrisons


The regiment was transferred from Dacia to Britain not later than 125, when it was stationed briefly at Fort Fanum Cocidi (Bewcastle, Cumbria) and appears to have participated in the excavation of the so-called Vallum, a huge ditch running along the near side of Hadrian's Wall (constructed 122-8). It was permanently stationed at Fort Banna (Birdoswald, Cumbria), on Hadrian's Wall, from 126 to at least 276/82, where it is attested in numerous inscriptions.

The inscription of 276/82 is the latest datable record found which mentions I Aelia Dacorum. However, a later inscription found at Fort Banna, datable to the latter period of the emperor Diocletian's rule (293-305), records that the fort's command structures, which had fallen into ruins, were rebuilt under the direction of the centurion Flavius Martinus, who is described as praepositus cohortis ("cohort acting-commander"). The fact that the fort was in ruins and then repaired suggests that it may have been abandoned for a period, and then re-occupied. But because the text is incomplete, it is uncertain whether Martinus commanded I Aelia Dacorum, or some other regiment.

The regiment is recorded in the Notitia Dignitatum, a late 4th-century official document which contains a list of Roman military units in existence around AD 400, as stationed at Camboglanna (Castlesteads, Cumbria), the fort on Hadrian's Wall neighbouring Banna to the east. This record shows that the regiment was probably present in Britain until the definitive withdrawal of Roman troops from the island in c. 410. By this time, the unit probably contained only c. 300 effectives, around a third of its size in the 2nd century. This reduction was in line with the army in Britain as a whole, which had shrunk from a peak of c. 55,000 in 200 to c. 17,500 effectives in 400.

Campaigns
In view of its size and long-term stationing on the northern British frontier, facing the unconquered Caledonia (Scotland), the regiment almost certainly participated in all the major campaigns recorded in this turbulent region, including:
 * 139-42: Antoninus Pius (r. 138-61) launches an aggressive stategy to re-occupy the Scottish Lowlands, as far as the Forth-Clyde line, which had been abandoned under the emperor Trajan (r. 98-117). Governor Quintus Lollius Urbicus leads the campaign. Coins record victory in 142/3 over the Caledonian tribes of the region, especially the Selgovae. The campaign is followed by the construction of the Antonine Wall.
 * 154-8: Serious disturbances break out in northern Britain, probably centred on a revolt by the Brigantes, most of whom reside south of Hadrian's Wall. The Romans are forced to withdraw troops from the Antonine Wall to suppress the revolt.
 * 181-5: The Caledonian tribes overrun Hadrian's Wall. The emperor Commodus (r. 180-92) rushes reinforcements under Lucius Ulpius Marcellus to repel the invasion. In 184, Commodus assumes the title Britannicus to celebrate victory.
 * 196-7: The governor of Britain, Decimus Clodius Albinus, launches a bid to seize imperial power. He leads the British army to Gaul to challenge the Danubian army under emperor Lucius Septimius Severus (r. 193-211). Frere argues that Albinus would probably have needed to take virtually every single Roman soldier from Britain for his campaign (his army reportedly numbered 150,000, but this is probably an exaggeration, as the entire army in Britain at this time probably totalled c. 50,000. Also, Albinus failed to win the support of the Rhine army). Albinus and his army were defeated in a hard-fought battle at Lugdunum (Lyon) and Albinus executed.
 * 208-11: Emperor Septimius Severus launches a massive campaign to conquer the whole of Caledonia, similar to that of governor Gnaeus Julius Agricola in 77-85, over a century earlier. However, from the evidence of Severan-era marching-camps along the east coast of Scotland, it does not appear that Severus' army advanced as far as north as Agricola's, which had reached Inverness. Severus' gains were abandoned by his son and successor Caracalla (r. 211-8).

Honours
The epigraphic record shows that, during the 2nd and 3rd centuries, the regiment won 5 more imperial titles, presumably for valour, or for loyalty to the awarding emperor:
 * 1) Antoniniana: this title, attested in an inscription dated 218-22, could, in view of the name, have been accorded by Antoninus Pius, Marcus Aurelius Commodus or Septimius Severus. However, it appears that in 212/3, several regiments in Britain were accorded the title. Frere argues that these awards were made by the emperor Caracalla (M. Aurelius Severus Antoninus, r. 211-17), in order to secure the British regiments' loyalty after the assassination of his brother and co-emperor Geta, who enjoyed wide popularity in the army.
 * 2) Gordiana (Gordian III r. 238 )
 * 3) Postumiana (Postumus r. 260-9)
 * 4) Tetriciana (Tetricus I r. 269-74)
 * 5) Probiana (Probus r. 276-82)

The awards show that, in the period 260-74, the regiment gave its allegiance to the "Gallic" emperors Postumus and Tetricus I, who ruled the breakaway western section of the Empire (comprising Gaul, Spain and Britain), known as the "Gallic Empire".

Religious affiliation
Of the 27 stone altars found at the cohort's long-term fort (at Birdoswald, Cumbria), 24 are deicated to Jupiter, the highest Roman god, originally the supreme sky-god of the Indo-Europeans. These are headed with the initials "IOM", for Iovi Optimo Maximo ("to Jupiter the Best and Greatest"). One of these was jointly dedicated to Jupiter and to the numen ("Divine Spirit") of the first emperor Augustus, who was officially deified after his death. One altar is dedicated to the Roman god of war Mars and one to the god of the sea, Neptune. Only one altar was dedicated to a native British deity, the Celtic god Cocidius (after whom the fort occupied by the cohort before Banna was named: Fanum Cocidi, meaning "Shrine of Cocidius", at Bewcastle, Cumbria).

However, these dedications are in no way representative of the religious affiliation of the regiment's members, as they were all erected by the cohort's tribunes (commanding officers). They thus represent the army's official cults. The cohort's soldiers doubtless revered a great variety of British and other native deities.