The number of legions in the empire (each composed of c.5-6,000 men) stayed much the same until well into the third century ce. However, a comparison of Tacitus’ account of their deployment in 23 ce (Ann. 4.5), and Dio’s (55.23.2-7; 24.2-4) description of the situation in his own day (early third century ce) reveals that their deployment changed over time. The increased emphasis on the Balkan provinces (such as Pannonia and Moesia), and a consequent shift away from the Rhine, is noteworthy, as is the relative size of the British garrison (Table 11.1). Marcus Aurelius raised two new legions (II and III Italica) for service in the Balkan provinces as pressure on that frontier mounted in the later second century ce, but the largest increase was Septimius Severus’ creation of three new legions with the title Parthica. Two garrisoned the new province of Mesopotamia. II Parthica was normally based at Albanum, just south of Rome, the first legion to be based in Italy for over two centuries. However, epigraphic evidence from Apamea in Syria shows that the legion was redeployed there regularly for imperial campaigns through the first half of the third century ce, suggesting it served as a central reserve (Balty 1988; Balty and van Rengen 1993; van Rengen 2000; Some of the inscriptions are reprinted in AE, esp. 1993 nos. 1571-91).
Through the second century ce, the frontiers of the empire became more settled and formalized in many regions. An obvious but cliched illustration of this is Hadrian’s Wall in the north of Britain, but the physical form of Roman frontiers varied from region to region. Even the development of walls and fortifications did not imply limits to Roman power or a defensive strategy. These fortifications were used as bases for offensive action and Roman power was maintained beyond them by both diplomatic and military means (Whittaker 1994; Elton 1996a). As part of this process, legions tended to settle down into long-term quarters in fortresses such as Eboracum (York), Ara Ubiorum/Colonia Agrippinensis (Cologne), Mogontiacum (Mainz), Vindobona (Vienna), Aquincum (Budapest), and Singidunum (Belgrade). Often they served as the nucleus of a developing civilian settlement, initially a dependent settlement with the status of canabae, but many (including some of the above, and Carnuntum in Upper Pannonia) evolved into proper cities with the status
Numbers are the number of legions attested in the province or region by that source
I Dio 55.23-4 reflects the situation before Septimius Severus’ creation of the three legions Parthicae, deployed in his new province of Mesopotamia (2) and in Italy (1).
Ii ILS 2288 (CIL VI.3492) is a column base from Rome originally inscribed with the locations of the legions in the reign of Marcus Aurelius, with the three new Severan legions added subsequently.
Iii ‘‘N’’ indicates that that province was not part of the empire (or was a client-kingdom) at that time.
Of municipium or colonia. On the eastern frontier legions often were based in or near existing cities, such as Zeugma in Syria and Jerusalem (Isaac 1992; Pollard 2000).
Instead of moving whole legions around the empire, it became a common practice to deploy detachments termed vexillations (after the vexillum standard they used). We certainly see substantial bodies of legionaries serving away from their parent legions by the later first century ce (Jos. BJ2.499; 5.43-4) and it was a common practice by the Antonine period. For example, an inscription shows that a senior centurion, T. Pontius Sabinus, led (as praepositus) thousand-man detachments from three German legions to campaign in Britain (ILS 2726). Vexillations might include attached auxiliary units or even consist entirely of them, as in the inscription CIL III.600, which lists six cavalry units (alae) and 15 auxiliary infantry cohorts led by a praepositus in Mesopotamia.
While, as Table 11.1 shows, most of the legions were based in frontier provinces, even the governors of traditionally ‘‘unarmed’’ provinces like Asia had detachments of legionaries (often commanded by centurions) and auxiliary units for administrative and internal security duties. There are numerous references to small groups of soldiers used for policing, as prison guards and under the command of other officials such as the procurator in the younger Pliny’s letters from Bithynia-Pontus (Pliny Ep. 10.17b-18, 19-20, 21-2, 27-8, 77-8).
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