We are no better informed about the causes of war in the period after Augustus, but we need to ask, first, how emperors made their personal decisions on questions of war and peace; second, why individual emperors committed the resources of the Roman world to war; and, finally, how far long-term strategic considerations may have affected the pattern of warfare.
All emperors after Augustus retained personal charge of the deployment of the army, and controlled decisions on war and peace. Our sources assume this without debate and are probably right. Tacitus explains how Tiberius personally decided not to pursue a war against the Frisii, a tribe living on the east bank of the Rhine, and instead suppressed information on Roman losses.51 The great wars of conquest in the first and second centuries - in Britain, in Dacia and Parthia, and in Mesopotamia - are all ascribed by the ancient writers to the personal decision of emperors.52 More strikingly, Trajan’s control of the supply of information on the Parthian War is demonstrated in his adoption of the honorary title ‘Conqueror of the Parthians’ (Parthicus) in 116:
On 20 (or 21) February a dispatch decked with laurel was sent to the senate by the emperor Trajan Augustus. For this reason he was named ‘Conqueror of the Parthians’, and for his safe deliverance a decree of the senate was passed, offerings were made at all the shrines, and games were carried on.53
Previously, in 102, Trajan had messengers from the Dacian king Decebalus sent on to the senate to confirm the terms of the peace treaty, which he had already negotiated. Later, when he adjudged Decebalus to have broken the treaty, the senate obediently declared the king a public enemy (hostis), and war began again.54 The prayer of the priestly college the Arval Brethren for Caracalla’s campaigns in the north in 213 also shows imperial responsibility for the detail of military policy: ‘11 August. . . because our lord, the most revered pious emperor M. Aurelius Antoninus Augustus, chief priest, is setting out to cross the border of Raetia and enter the [land] of the barbarians in order to annihilate the enemy, may this turn out fortunately and luckily for him.’ 55
Emperors, of course, continued to take informal advice from their amici, but it is hard to say how decisive this was. Our sources were poorly informed about council meetings, which will have normally taken place in secret. However, the satirist Juvenal gives us a comic version of what he imagined such a meeting was like in the reign of Domitian. The advisers were summoned in haste (‘Hurry! The Emperor is seated’), and arrived nervously fearing a military disaster - ‘panic-stricken dispatches might have been pouring in from all parts of the empire’. In fact they were asked to suggest ways of cooking a large fish, and duly gave their advice.56 A more serious debate took place under Nero on the situation in Parthia. After a military setback he consulted his council of advisers with the question: Should they accept a humiliating peace or risk a hazardous war? They immediately decided that they must go to war.57 On the other hand, after the death of Marcus Aurelius, his son Commodus ignored advice from his father’s advisers and made peace with the Marcomanni, although the opinion was that the Romans could have destroyed them.58
To go to war was a serious decision for a Roman emperor, and we need to know what factors influenced his decision. There were many reasons for hesitation: inertia, precedent and the importance of the status quo, which might tell against extravagant military decisions. In most aspects of government, emperors usually responded to events and pressure from below, and those who took the initiative, or who were compelled to do so, stand out in our sources.59 Then the expenses of a campaign might or might not be recouped by the profits of victory, and there were potential logistical difficulties in assembling large numbers of troops for military operations, given the static nature of the dispositions of the Roman army. On the other hand, it is not clear to us how an emperor could calculate or oversee these matters, or even if he thought that they were important.
On a sinister note, some emperors might want to avoid war to prevent others from acquiring the glory associated with military leadership. Senators still commanded the legions as part of their traditional duties as provincial governors, but emperors could ensure that they were denied the responsibility of an independent command in a major campaign. For example, Claudius prevented his governor of Lower Germany, Domitius Corbulo, from crossing the Rhine to engage the Germans. Corbulo wryly commented: ‘once upon a time Rome’s generals were lucky!’ Indeed, Pliny believed that jealousy and fear of imperial displeasure in the reign of Domitian paralysed military commanders.60
An emperor could also restrict the role of senators by taking personal command of major campaigns, as generally happened from the late first century onwards. However, in the two centuries after the death of Tiberius, most emperors had little or no military experience, except for Galba (who ruled for only one year), Vespasian and his son Titus, Trajan and Hadrian. Marcus Aurelius and Lucius Verus, who by circumstance were forced to spend many years fighting in the east and the north, had no military experience whatever. Furthermore, up to the reign of Septimius Severus the reigning emperor rarely had a member of his family on whom he could rely to take charge of major wars. An exception was Vespasian, who did indeed employ Titus to stamp out the Jewish rebellion. Therefore an inexperienced emperor on campaign might be anxious about the possibility of defeat and its repercussions, notably the potential damage to his image, which could help to undermine his standing in Rome. For defeat would be his personal responsibility and could not easily be blamed on one of his generals.61 Furthermore, it might be wise to avoid long wars, since it has been plausibly suggested that over time the will to war fades as the costs mount up; people do not like living in perpetual fear, and protracted war can lead to internal divisions. Of course, once a decision had been taken either to wage war or to seek a resolution by other means, the government could rationalize this and make a suitable presentation for public consumption.62
There was also possible political danger in leaving Rome for long periods in order to conduct military campaigns. Rome was the centre of the nexus of administrative, political and social contacts that helped to underpin the imperial position, and in the capital there was a well-established pattern of protecting the emperor through the praetorians and the urban cohorts. Tiberius, declining to leave Rome to deal with the mutinies of ad 14 on the Rhine and the Danube, vividly described it as the ‘head of everything’.63 Tacitus recognized this when he made events at Rome the centrepiece of his narrative in the Annals. In the emperor’s absence from Rome, dissension or even revolt could perhaps be more easily fomented.
Emperors who were nervous of command, or preoccupied with administrative duties, or indifferent to conquest, could find an alternative to war by pursuing and developing the relatively sophisticated diplomatic contacts that had been established by the late first century AD. These involved envoys of high status, formal meetings and dinners, negotiation, written treaties and recognition of the importance of good faith. Of course, diplomacy generally meant that the Romans got what they wanted (or a convincing version of it) without fighting, and had proved particularly effective in dealing with the Parthians.64
We might therefore think that emperors would need strong reasons to commit Roman troops to lengthy campaigns, but three other factors are relevant. First, the military trappings of the emperor’s position, his titles and attributes, and his depiction in art, which owed much to decisions taken by Augustus, all contributed to a warlike image that never faltered through the first three centuries. Some emperors may have felt the need to live up to this and acquire military honours to enhance their prestige and that of their house.65
Second, the psychology of war in Rome was important. Military attributes remained common in society and culture, and war was never likely to be thoroughly unpopular. It could be said of imperial Rome that ‘war was a noble and necessary activity for any state desirous of demonstrating its power and virtue’.66 The Romans continued to be impressed by military success and an ability to master the qualities and attributes of a general. They were interested in both the technical and moral aspects of commanding men in battle, as we can see from the many handbooks and guides written about the character and role of a general.67 It is interesting that Appian’s history of Rome is arranged in a series of war narratives according to the various peoples whom the Romans had fought and conquered. Presumably he expected that this demonstration of how the Romans ‘acquired unparalleled foreign domination, and brought the greater part of the nations under their control’ would appeal to his audience.68 Florus, writing in the mid-second century ad, composed a work entitled Abridgement of All the Wars of Seven Hundred Years to encapsulate the history of the Roman people, who had carried their weapons throughout the world, so that their exploits seemed like the history not of a single people but of the human race.69 More substantial historians, like Tacitus and Cassius Dio, dealt seriously with warfare, the status of Rome and relations with foreign peoples. The fact that senators like these despised the ordinary soldier does not mean that they despised military command itself. Indeed, Dio commanded two legions as governor of Upper Pannonia.70
This interest in warfare often brought with it an implicit assumption that the Romans’ military superiority was absolute and that they could arrange peoples and kingdoms to suit their interests, and continue conquering when and where they wished.71 To take one example, Livy in his history recounted with many stirring anecdotes Rome’s glorious military heritage, and in the Preface wrote that the Roman people’s military glory was so great that it was reasonable for them to think that they were descended from Mars, the god of war.72 The story of the reappearance in a vision of the first king, Romulus, was certainly fictitious, but his message to the city reflected the reality of what Romans thought about their empire: ‘my Rome shall be the capital of the world; so let them foster the art of war, and let them know and hand down to their children that no human strength can resist Roman arms.’73 It is hardly surprising that emperors gave expression in word and deed to these long-established feelings. Thus Tiberius, no warmonger, was furious because of his generals’ inability to end the war in Africa against Tacfarinas, who had sent envoys to the emperor with demands. ‘No insult to him or the Roman people, it is said, ever annoyed the emperor more than that this deserter and brigand should behave like a hostile king.’74 Such sentiments did not abate with the passage of time. In ad 375 the emperor Valentinian received ambassadors from the Quadi, who came to meet him at Brigetio (Szony) to negotiate a treaty, but in private audience they defended their previous conduct so insolently that the emperor in a rage suffered an attack of apoplexy and died soon afterwards.75
Third, a Roman emperor had few constraints upon him. There were lands and peoples where it was feasible for the Romans to seek further conquest with a good expectation of victory. At least before the mid-third century there was no shortage of funds, supplies and recruits to support military expansion. Furthermore, since the army was becoming gradually less Italian in composition, there will have been correspondingly less pressure from public opinion about the possible loss of Italian lives on needless campaigns.76 The emperor was commander-in-chief of a large army, but there is little sign that any emperor was greatly concerned about soldiers’ opinions on military matters, or came under pressure from the soldiers for action and booty.77 Mutinies about imperial military policy rarely occurred, or at least are rarely reported, and most of our information concerns random disturbances related to the conduct of individuals, or isolated military incidents.78 Although in the serious mutinies of ad 14 an important factor was the soldiers’ concern about what would happen to them after Augustus’ death, in my view the underlying impetus was discontent with pay and conditions. In general, although emperors identified themselves closely with the army, they remained aloof from and even contemptuous of the ordinary soldiers, and seem to have felt able to devise military policy free from any significant concerns.
This is the background against which individual emperors decided to go to war. The empire was an autocracy, and the character and behaviour of emperors are important factors in imperial politics, but we are poorly informed about precisely why emperors took decisions. As Cassius Dio points out, it was often difficult to get information about what happened in the provinces and on the periphery of the empire.79 When ancient writers do give a specific reason for a war, they usually emphasize an emperor’s desire for glory, renown and personal prestige. This is Dio’s explanation of Trajan’s Parthian war, and he must have thought that this was at least plausible.80 Trajan had had an unusually long period of service as a military tribune, and was governor of Upper Germany when he became emperor. He liked to be seen as a soldier’s man, participating in military exercises, marching at the head of his men on campaign, and allegedly tearing up his own clothing for bandages during the Dacian Wars.81 Now, although Trajan had a reasonable pretext for his invasion of Dacia, in the apparently unsatisfactory nature of Domitian’s settlement with the Dacian king, Decebalus, he was perhaps attracted by the idea of waging war on a seemingly powerful and arrogant people on the periphery of the empire.82 His success probably accustomed him to the military life, which he tried to re-create with the war against Parthia. All the emperor’s military exploits brought him extraordinary honours and were celebrated in art and architecture, most notably in the column that adorned his forum.83 Therefore Dio’s explanation rings true, though we cannot be sure what other ideas were in Trajan’s mind, and hopes of plunder and loot cannot be discounted. However, the emperor wanted more than merely to conquer people and amass booty, since he annexed Dacia as a province, and then Arabia (probably in 106), and later was apparently planning to create new provinces beyond the Euphrates.84
In the aftermath of the murder of Gaius, Claudius had to rely on the embarrassingly open support of the praetorians as he plotted to become emperor. He was physically unappealing, suffered from a stammer and was unpopular among the upper classes. There seems little doubt that an important motive for the invasion of Britain in ad 43 was the exploitation of the glory that came with military success to enhance Claudius’ political standing. Suetonius was sure that Claudius ordered the invasion of Britain because he wanted a campaign where he could earn a proper triumph. He therefore had to go in person, and the invasion commander Aulus Plautius, as instructed, summoned him at a crucial moment of the campaign, although he stayed in Britain for just sixteen days. On his return to Rome, amid massive celebrations he ceremonially extended the formal boundary of the city (pomerium) symbolizing the addition of new territory, which even Augustus had not done, and honoured many of the upper classes who had served in the invasion.85
From AD 82/3 Domitian embarked on a series of wars on the Rhine and the Danube. Rome was encountering difficult relations with peoples in this area, but the emperor was probably not unhappy to see war and the opportunity for military glory. He had poor relations with the senate; and, in AD 89, L. Antonius Saturninus, governor of Upper Germany, revolted, so that the loyalty of the army was severely tested, and perhaps was only partly confirmed by a timely pay rise. Domitian himself was short of military prestige in comparison to his father and brother Vespasian and Titus, both of whom had distinguished military reputations.86
After his seizure of power in ad 193, Septimius Severus fought two bitter civil wars, which brought huge Roman casualties and much resentment among the upper classes. He needed a respectable war against foreign enemies, and this is probably the reason for the two campaigns he conducted in the east. Since the reign of Trajan, Rome’s relations with Parthia had been unstable, and that land always offered a venue where limited wars could be fought and military glory gained in the footsteps of Alexander the Great. Once again there were enormous celebrations in Rome. Interestingly, Severus tried to explain his Parthian war on strategic grounds, arguing that the land acquired would be a defence for the province of Syria; but in the view of his contemporary, the historian Dio, Rome was now embroiled unnecessarily in expensive conflict with peoples on the periphery of the empire. Then, late in his reign, Severus led a campaign in Britain; and, although there was a pretext of some turbulent British tribes, ancient writers believed that the real motive was personal and political - namely, to give his sons something honourable to do and to procure dynastic stability.87
Warfare did not always involve great campaigns of conquest. Apart from low-level violence, where the army was involved in putting down rebels, disposing of bandits and keeping order, the preservation of the territorial integrity of the Roman empire was itself an important motive for war. It was in an emperor’s self-interest to ensure that land long recognized as Roman was not overrun or abandoned, since this, too, could have political implications. Criticism of an emperor’s foreign and military policy could easily become criticism of his capacity to rule.88 Hostile commentators alleged against Nero and Hadrian that they toyed with the idea of giving up Britain and Dacia respectively.89 In fact, emperors were prepared to fight to retain or consolidate such peripheral areas. In the second century Britain detained three legions and perhaps a total of 50,000 troops in its garrison.90
But perhaps we should not see this as entirely a matter of Roman imperial self-interest. Some emperors perhaps really did want to ensure the security, peace and well-being of the inhabitants of the empire.91 Emperors did, after all, respond from time to time to deputations and appeals from their subjects, both individuals and communities. Velleius, a strong supporter of the imperial regime, was probably quite genuine when he celebrated the return of Tiberius to army command as being for ‘the defence of the empire’.92 Frontinus believed that by defeating the Germans Domitian had acted for the benefit of the adjacent Roman provinces.93 Plautius Silvanus Aelianus, Nero’s distinguished governor of Moesia, can speak for himself in the inscription in which he celebrates his career, and lists various military and diplomatic achievements ‘by which he confirmed and extended the peace of the province’.94 Much later the emperor Constantine was to be celebrated by the community of Tropaeum Traiani as a ‘restorer of Roman security and liberty’.95 Other writers speak of the army as a kind of protective ring around the empire. Appian describes how the Romans ‘surround the empire with large armies and garrison all this land and sea like a single fortress’.96 Similarly, Tacitus summarizes the situation in ad 14:‘The empire was fenced round by the ocean or distant rivers; legions, provinces, fleets were all linked to one another.’97 Of course a fear (real or imagined) of powerful neighbours, who might or might not threaten Roman territory and subjects, could lead to a pre-emptive attack by the Romans, which could then become a campaign of conquest. Marcus Aurelius began his northern wars as a response to incursions by German tribes, but at the end of his life seemed to be contemplating the creation of a new trans-Danubian province of Sarmatia.98
World History
