Gallic Wars 2.25.
22 On Scaeva, see Suetonius Caesar 68.3–4; Appian Civil War 2.60; Dio mentions a Scaevius who served with Caesar in Spain in 61 BC, Dio 38.53.3. For the ala Scaevae
CIL 10.6011 and comments in J. Spaul, ALA2 (1994): 20–21. For the social status and levels of education among centurions, see J. N. Adams, “The Poets of Bu Njem: Language,
Culture and the Centurionate,” Journal of Roman Studies 89 (1999): 109–34.
23 Ronald Syme, The Roman Revolution (Oxford: Clarendon Press, 1939), 70, 78–79; on
the execution of unruly soldiers, see Dio 43.24.3–4.
24 Suetonius Caesar 77, 86.
226 Goldsworthy
10. Holding the Line
Frontier Defense and the Later Roman Empire
Peter J. Heather
According to an analysis first offered by Edward Luttwak in
the mid-1970s, the Roman Empire consciously moved from a fron-
tier policy based on expansion to one based on defense in depth from
the Severan era at the start of the third century AD. From this point
on, its military effort was directed toward strategically planned belts of
fortifications designed to absorb small-scale threats, backed by mobile,
regionally based field armies held in reserve and carefully placed to deal
with larger-scale incursions.1 In the summer of 370, for instance, some
Saxon raiders used ships to avoid the frontier defenses of the northern
Rhine and landed in northern France. Substantial raiding followed, un-
til the local Roman commander gathered sufficient heavy cavalry and
infantry units to ambush and destroy the now unsuspecting Saxons,
who had been lulled into a false sense of security by a truce that osten-
sibly permitted them to withdraw unharmed.2 This is a textbook ex-
ample of the kind of frontier strategy Luttwak identified, but on closer
inspection, and despite the continuing influence of his work, which
has remained solidly in print for more than thirty years, his analysis is
substantially mistaken.
For one thing, while successive moments of energetic activity along
the frontier are detectable in the archaeological record, some of which
affected the many thousands of kilometers separating the mouth of
the Rhine from that of the Danube, campaigns and fortress building
can sometimes be shown to have had rather more to do with inter-
nal political agendas than with rational military planning. Keeping the
barbarians at bay was the fundamental justification for the large-scale
taxation of agricultural production that kept the empire in existence.
Not surprisingly, emperors liked to show the landowners, who both
paid and levied these comparatively vast sums of annually renewable
wealth, that they were tough on barbarians, and tough on the causes
of barbarism. In the 360s, for instance, the brother emperors Valentin-
ian I and Valens built fortresses energetically on the empire’s Rhine and
Danube frontiers to make the point that they were taking proper care
of the empire, even though the policy broke some agreements with
frontier groups that were currently peaceful.3 Valentinian also unilater-
ally lowered the annual subsidies being paid to some Alamannic leaders
on the Upper Rhine, in order to be able to claim that he did not buy
peace from barbarians.4 Both lines of policy were highly irrational in
terms of maintaining frontier security, because they actually provoked
disturbances, but the emperors’ internal political agendas came first.5
Offensive warfare likewise had not come to end, more or less at the
end of the third century, because of any carefully planned, strategically
informed decision making, based on the rational analysis of the capac-
ity of the empire’s economy to generate sufficient forces to defend its
existing assets. Rather, further attempts at conquest had slowly run out
of steam on all of Rome’s frontiers on a much more ad hoc basis when
it became all too apparent that the fruits of conquest—usually mea-
sured in terms of the glory generated for individual rulers rather than
any rational, strategically minded cost-benefit equation—ceased to be
worth the effort.6
Politicians’ egos and internal political agendas have long interfered
with rational military planning, however, and it should come as no
great surprise that this was also true in the ancient world. Arguably,
therefore, a much greater deficiency in Luttwak’s analysis is his lack
of attention to how Rome’s frontier assets—the combination of for-
tifications and troops he so insightfully identified—were actually used
in practice in the late Roman period of the third and fourth centuries.
For right through to the end of the fourth century, Roman forces did
not merely wait for the barbarians, sitting behind belts of formidable
frontier fortification like some equally doomed precursor of the French
and the Maginot Line. Such a sequence of events did unfold along the
228 Heather
frontier on occasion, as in the case of the Saxons in 370, for instance,
but far too infrequently for it to count as the dominant strategy that
the Romans employed for maintaining frontier security. Such a strategy
would not in any case have proved very effective. To economize on
pay, equipment, and supplies, many units of the mobile armies were
kept at such a low state of readiness unless a campaign was actually
in the offing, and slow speeds of movement made for such slow re-
sponse times, that there was a substantial danger that even quite large
barbarian raiding forces would be safely back across the frontier long
before any effective counterstrike could be launched. Everything ex-
cept messages could move no faster than about 40 km per day, and to
ease the problem of supplying them, even the mobile troops were not
quartered in very dense clusters. To concentrate a decent force against
any attack and then get the troops to a point where they could actually
intervene was generally a matter of weeks rather than days, so that a
purely responsive strategy would always leave raiders with plenty of
opportunity for pillage and withdrawal.7 I suspect, in fact, that it was
precisely to buy the time he needed to mobilize sufficient troops that
the local Roman commander went through the sham of concocting his
pseudo-agreement with the Saxon raiders of 370.
When the narrative historical sources are added to the evidence of
military archaeology and known troop deployments, a very different
picture of overall Roman strategy emerges. Forts and armies were
only two elements of an approach to frontier management that relied
extremely heavily on a manipulative repertoire of diplomatic intru-
sions, backed by the periodic deployment of main force. The typical
pattern that emerges from late third- and fourth-century sources is
that whenever the political-military situation along a particular frontier
zone threatened to get out of control, a major campaign, often led
personally by a reigning emperor, would be mounted beyond the impe-
rial border. The Tetrarchic emperors of the late third and early fourth
centuries mounted a succession of such campaigns on a
ll three main
sectors of Rome’s European frontiers: the Rhine, Middle Danube (west
of the Iron Gates), and Lower Danube (to the east). Constantine I cam-
paigned on the Rhine in the 310s, and in the Lower and Middle Danube
regions in 330s. His son Constantius II (together with his cousin and
Frontier Defense 229
caesar Julian) led armies east of the Rhine and north of the Middle
Danube in the 350s, while in the next decade Valentinian and Valens
again led substantial military forces over the frontier in both the Rhine
and Lower Danube regions.8
As recounted in the surviving narrative sources, these campaigns
tended to follow a similar script. Any overly mighty barbarian leaders
were first subdued, and then, for a period, the Roman armies set to
burning down every settlement they could find.9 But the deeper pur-
pose of this kind of military action was not destruction per se, although
the campaign certainly had a deliberately punitive purpose, as well as
being good for military morale, since it allowed the troops to pillage at
will. The effects of regular Roman pillaging of frontier areas are also
occasionally visible archaeologically.10 Nonetheless, the actual Roman
campaigning was no more than the precursor to the campaigns’ main
purpose, which was to force all the higher- and medium-level barbarian
leaders of the affected region formally to submit to imperial authority.
Once a sufficiently dominant display of aggression had been deemed
to have occurred, the emperor would typically establish his camp, and
the regional barbarian leaders would troop in one by one to make their
submissions. This process is again explicitly described on a number of
separate late Roman occasions, from the Tetrarchic emperor Maximian
in the 290s to Constantius II and Julian in the 350s and beyond.11 How
far into barbarian territory beyond the frontier these campaigns tended
to range is not clear. They could certainly last several weeks, however,
and I suspect that at least their diplomatic effects—in the form of ac-
quisition of territories belonging to the procession of submitting kings
and princes—were felt for a distance of about 100 km beyond the impe-
rial frontier.12
The emperor and his advisers then set about turning short-term
military dominance into longer-term security, as the example of Con-
stantius II’s manipulations in the Middle Danube region in the 350s
effectively illustrates. First on the agenda was a survey of current bar-
barian political confederations in the affected region. If any were too
large, posing too great a threat to frontier security, they were broken
up, with subleaders being handed back their independence from the
overly mighty king to whom they currently owed allegiance. In the
230 Heather
Middle Danube of the 350s, for instance, Constantius was clearly wor-
ried about the power of a certain Araharius, and freed some Sarmatian
vassals led by Usafer from his control. He also elevated the prince of
another group of Sarmatians, Zizais by name, to royal status and re-
newed the political independence of his following. This independence
was backed by guarantees of Roman military support and reinforced
by targeted annual diplomatic subsidies to reinforce more favored
leaders’ positions. It has sometimes been argued that these subsidies
were “tribute” and a sign of late imperial weakness. As a diplomatic
tool, however, they had been in constant use since the first and second
centuries, when Roman dominance was pretty much absolute, and in
the fourth century they were granted even to barbarian leaders who
had submitted. They should clearly be understood in modern terms,
therefore, as targeted aid, designed to shore up the power of Rome’s
chosen diplomatic partners.13
While all this diplomatic maneuvering was under way, any Roman
captives held in the region were released, and forced drafts of man-
power were taken from the submitting barbarians as recruits for the
Roman army.14 At the same time, these periodic emperor-led interven-
tions were usually undertaken in response to some reasonably sus-
tained bout of frontier trouble. Because campaigning on this scale was
expensive, it usually required a major sequence of disturbances to act
as a trigger. Even aside from the Roman troops’ pillaging, therefore, it
was entirely common for the renewed agreements to contain punitive
clauses, with those held to be the guilty parties being punished in a
variety of ways. On occasion, this could even mean a barbarian king’s
execution. In 309, for instance, Constantine I executed two kings of
the Franks in the theater at Trier.15 More usually, however, the desire
for revenge was satisfied by imposing various financial penalties, most
commonly in the form of exactions of labor and raw materials for re-
building work, along with substantial quantities of food supplies.16
From time to time, emperors might use their military superiority to
take much more drastic action. Aside from breaking up dangerous polit-
ical structures on the other side of the frontier, emperors were also con-
cerned to ensure that no frontier region became overcrowded. This was
a situation that could and did lead to the internal rivalries of different
Frontier Defense 231
barbarian leaders spil ing over onto Roman soil. One response was to
force—at sword point, if necessary—some of the empire’s immediate
neighbors to abandon their established homes and move away from the
frontier zone. In the Middle Danube of the 350s, for instance, Constan-
tius II decided that one particular Sarmatian subgroup, the Limigantes,
was to be expel ed, and was entirely happy to use force to make them
leave. Another response was to al ow particular barbarian groups to be
received onto Roman soil on strictly regulated terms. The Tetrarchs in
particular employed such control ed resettlements on al the major Eu-
ropean frontiers in the two decades after 290 AD, but this technique had
an established prehistory and continued in use subsequently.17
This is not to say that Rome’s repertoire of manipulative diplomatic
techniques was always deployed as part of an entirely coherent or ra-
tional policy for frontier defense in what might be termed a “grand
strategy,” with the emphasis firmly on “grand.” As we have seen, in-
ternal political agendas sometimes made emperors pick fights where
there was no need, so that they could show off to their taxpayers. In
reality, too, all the major cross-border campaigns of the late imperial
period tended to be responsive, coming after the breakdown of order
within a particular frontier region, rather than because political and
military intelligence indicated that order was about to break down.
When assessing the overall effectiveness of Roman frontier defense,
therefore, it is necessary to factor into the equation that substantial
economic losses to outside raiding were also part of the picture, since
it took a fair amount of raiding to trigger a response. How substantial
; that raiding might have been has emerged from an exciting archaeolog-
ical find made while dredging in the Rhine near the old Roman frontier
town of Speyer. Late in the third century, some Alamannic raiders had
been trying to get their booty back home across the Rhine when their
boats were ambushed and sunk by Roman river patrol ships. This booty
consisted of an extraordinary 700 kg of goods packed into three or four
carts, the entire looted contents of probably a single Roman vil a, and
the raiders were interested in every piece of metalwork they could find.
The only items missing from the hoard were rich solid silver ware and
high-value personal jewelry. Either the lord and lady of the house got
away before the attack or else the very high-value loot was transported
232 Heather
separately. In the carts, however, was a vast mound of silverplate from
the dining room, the equipment from an entire kitchen (fifty-one caul-
drons, twenty-five bowls and basins, and twenty iron ladles), enough
agricultural implements to run a substantial farm, votive objects from
the vil a’s shrine, and thirty-nine good-quality silver coins.18 If this haul
represents the proceeds of just one localized raid, the magnitude of
the more sustained disturbances required to trigger an imperial cam-
paign should not be underestimated. Nonetheless, the overall pattern
of the evidence is unmistakable. Late Roman emperors did not leave
their troops passively behind the frontier merely waiting for trouble.
Periodically, the field armies were trundled out in force to establish an
overwhelming level of immediate military dominance, which was then
used to dictate an overall diplomatic settlement for the region that was
in line with the empire’s priorities, to maximize the cost-value ratio of
the original campaign.
In addition, a further subrepertoire of intrusive techniques was then
used to shore up each diplomatic settlement and increase its effective
life span. Targeted annual subsidies, designed to keep favored kings in
power, were common. Particularly favored groups would also receive
special trading privileges. Normally, trade was allowed only at a few
designated points in any frontier segment, but occasionally the empire
would throw in an open frontier to sweeten a deal. After his defeat in
Makers of Ancient Strategy: From the Persian Wars to the Fall of Rome Page 36
