The auxiliary forces provided the Roman imperial army with most of its cavalry, along with infantry units, including some specialists like archers, of lower status than the legions. Most auxiliaries were recruited from peregrini, free non-citizen inhabitants of the empire, who received citizenship on their retirement, in contrast to the largely citizen legionaries. However, individual Roman citizens joined auxiliary units too, and (as unit titles show) some units were raised from Roman citizens or rewarded with citizenship during their service. The distinction may have grown increasingly blurred, and the Constitutio Antoniniana made it meaningless.
Auxiliary infantry were organized in cohorts, most, notionally, 500 strong (quin-genaria/quingenary), but in practice rather smaller. From the later first century ce onwards we also know of larger cohorts, notionally of 1,000 men (mi//iaria/milli-ary). There were also cohorts described as equitatae (mounted), composed mostly of infantry, but with a detachment of cavalry. One such unit was the 20th cohort of Palmyrenes, based at Dura-Europos in Syria. An early third century papyrus document (P. Dura 82 = Fink 1971: no. 47) shows that on one particular day the strength of this milliary cohort was 923 troops, including 223 cavalry and 34 camel riders. Auxiliary cavalry was organized in units called a/ae (wings), also quingenary or milliary (Le Bohec 1989b; Holder 1980; Cheesman 1914).
By the end of the first century ce most auxiliaries were part of the regular, professional Roman army, with individuals serving under Roman officers. As mentioned above, this regularization may have begun under Augustus, but apparently it developed through the century. At first, some auxiliary troops were still irregular levies serving under their own leaders. Batavians of the Lower Rhine, who were excused direct taxation because of their importance as soldiers (Tac. Hist. 4.12; Germ. 29), probably were among the ‘‘hurriedly raised bands of Germans’’ contrasted with regular auxiliaries in the army of Germanicus in 15 ce (Tac. Ann. 1.56). By the middle of the first century ce, Batavians were serving in regular cohorts in Britain, but still under their own leaders (Tac. Hist. 4.12). Civilis, a Batavian tribal leader and former Roman auxiliary officer, led a revolt in 69 ce (Tac. Hist. 4.32). In 68 ce, some of the auxiliaries in Vitellius’ army (largely drawn from the Rhine frontier) were recognizable by their ethnic names and native weapons (Tac. Hist. 2.89).
As mentioned above, Tacitus implies that the total number of legionaries in the empire was roughly the same as the number of auxiliaries (Ann. 4.5). This should be taken only as a rough estimate, with significant variation from region to region and over time. For example, it is estimated that the Hadrianic auxiliary garrison of Britain consisted of 14 a/ae and 47 cohorts (seven of them milliary), almost double the number of men in its three legions (Holder 1982: 17). To establish which auxiliary units were based in each province, we use epigraphic evidence, including military diplomas, discharge documents inscribed on bronze tablets to certify the status of retired auxiliary troops (as well as praetorians and sailors). These useful documents (collected in CILXVI and a series ofvolumes edited by Margaret Roxan 1978, 1985, and 1994; Roxan and Holder 2003), when intact, provide a date, the name of the individual discharged, his place of origin, the province he served in, and the name of his unit. The names of auxiliary units typically include an ethnic title, usually denoting where the unit originally was recruited. For example, ala III Augusta Thracum was a cavalry unit originally raised in Thrace, cohors IV Gallorum an infantry unit raised in Gaul, and cohors I Hamiorum sagittaria was a cohort of archers raised at Hama in Syria. Most auxiliary units moved away from the province in which they were first recruited. For example, Gemellus, son of Breucus, a Pannonian, served in the first ala Tampiana of Pannonians stationed in Britain. He was discharged in 122 ce, and apparently retired back to Pannonia, as his diploma (CIL XVI.69) was found in modern Hungary.
Military diplomas also list other units in the province that discharged veterans at the same time as the bearer of the diploma. An individual diploma rarely provides us with the identities of all the auxiliary units in a given province, but often they can be patched together to give a likely total. For example, the eight cavalry alae and 18 infantry cohorts listed in Syria in two texts issued on the same day in 88 ce ( CIL XVI.35; Roxan 1978: no. 3) perhaps represent the complete auxiliary garrison of the province, which also had three legions.
As the Roman army defended increasingly well-defined frontiers through the second century, we see evidence of auxiliaries deployed in a dispersed manner in the frontier zones, engaged in patrolling, policing, and other duties. For example, a (90s CE) unit strength report of the first cohort of Tungrians, preserved on a writing tablet from the fort of Vindolanda in Britain ( Tab. Vindol. II 154 = Campbell 1994: no. 182), records that this milliary unit had an actual strength of 752 men. However, only 296 of these were present (and only 265 healthy) at the fort. A further 337 were at Coria (Corbridge), 46 are described as singulares (probably detached to the governor’s guard in London), and the rest were at six other locations. Early third century papyrus duty rosters from Dura-Europos present a similar picture. P. Dura 100 (= Fink 1971: no. 1) lists men of the 20th cohort of Palmyrenes at no fewer than six locations besides Dura itself as well as on the governor’s staff, escorting the emperor and (perhaps) hunting lions.
While auxiliary troops often were dispersed throughout the frontier zone, legions were perhaps more concentrated, sometimes based a little to the rear, in reserve (Luttwak 1976: fig. 2.2; 2.4 for an abstract depiction of such a frontier scheme). For example, in Britain Hadrian’s Wall was mostly manned by auxiliaries while the three legionary bases were further south, at York, Chester, and Wroxeter. However, the traditional view of separate deployment and basing has been undermined, for example, by finds of apparently legionary equipment in supposedly auxiliary forts (Bishop and Coulston 1993: 209), and there is certainly epigraphic evidence for legionary vexillations serving alongside auxiliaries in frontier areas of Britain.
In addition to regular auxiliary troops, there were also forces provided by client kings, particularly in the eastern provinces in the first century ce. For example, at the start of the First Jewish Revolt, Cestius Gallus, the governor of Syria, had 14,000 men (mostly archers and cavalry) provided by three local rulers in addition to a full Roman legion, three legionary vexillations of 2,000 men each, six auxiliary cohorts, and four alae (Jos. BJ 2.500-1). Subsequently, we see ethnic units (often called numeri - misleadingly, since this term is applied much more widely) serving alongside regular Roman legionaries and auxiliaries (M. P. Speidel 1975). These troops fighting, initially at least under their own leaders in their native manner, included the Moors who fought under Lusius Quietus for Trajan (HA Had. 5.8) and subsequent Roman armies into the third century ce (Herod. 3.3.4-5; 6.7.8; Dio 78.32), and the Palmyrenes who served over much of the empire in the same period.
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