The Roman Army, 31 BC–AD 337: A Sourcebook

232 The Roman Army
military organization to changing circumstances. Gallienus began
regularly to employ men of equestrian rank in legionary commands
previously held by senators, and since these equestrians generally had
more military experience than senators, it is not surprising that the
military responsibilities of senatorial governors were gradually taken
over by equestrians, and the old constraint of the senatorial monopoly
of high office was at last removed. Moreover, the enhanced role of
cavalry and the increased use of detachments of troops for special
purposes, fostered the emergence of a kind of central force, containing
cavalry and infantry, which was able to move to the support of the
troops permanently stationed in individual provinces. Since it attended
normally on the person of the emperor and was commanded by him, it
might also make him less susceptible to revolt. The appearance of this
force, which was a precursor of the later field army, is unlikely to have
been the result of a single decision. It was put together originally to
deal with an immediate crisis, but in the course of time as its value was
appreciated it acquired a more permanent structure. Aurelian brought
additional resources to the army by recruiting fighting men from peoples
who had fought against Rome, like the Vandals.
Diocletian (AD 284–305) established the tetrarchy, in which the
emperor and his co-emperor were supported by two Caesars, each one
of the four in practice taking major responsibility for a geographical
area of the empire. This more stable military structure allowed
Diocletian to make use of the changes in the army made by his
predecessors and to introduce further reforms. While preserving the
independent field force, he also believed in the maintenance of a strong
military presence in the frontier zones of the empire, with legionary
and auxiliary, troops permanently stationed in the provinces. By AD
305 there were at least sixty-seven legions in service, that is, more than
double the number of troops serving in AD 235, if the legions had their
usual complement, although the evidence for this is inconclusive.
Diocletian also built more forts to ensure good communications by
road or river, to provide a base for defence and consolidation, and also
a platform for attack or counter-attack. The most notable example is
the Strata Diocletiana on the eastern frontier, where a chain of forts at
twenty-mile intervals garrisoned by infantry cohorts and some cavalry,
was linked by a military road; legionary bases were positioned in the
vicinity. The object was to guarantee Roman control in the area and
also to ensure communications from southern Syria to the Euphrates.
Diocletian naturally aimed to preserve Roman territory from outside
attack, but his military arrangements did not draw a merely defensive
line and did not rule out wars of aggression.