The foundation of the Parthian legions did, however, lead to changes in the expeditionary forces, particularly their overall command structure. The legio II Parthica was designed to accompany the emperor on campaign, a role it performed during Septimius Severus’ two Parthian wars and his British expedition. The question of whether the legion came under the direct command of the praefectus praetorio is a vexed one. In Cassius Dio’s Roman History the character Maecenas advises Octavian that the praetorian prefect should control all the forces stationed in Italy, a statement that could be taken refer to the situation in Dio’s own lifetime. As an official imperial comes during Severus’ Parthian campaigns, the prefect Fulvius Plautianus certainly joined the emperor in the east, but he is not mentioned in any specifically military capacity, in contrast with the abundant evidence for Severus’ senatorial generals leading troops in battle. It seems likely, therefore, that the authority of the praetorian prefect over the legio II Parthica evolved gradually. During Caracalla’s campaign against the Parthians his expeditionary force was composed of the legio II Parthica, the cohortes praetoriae, and the equites singulares Augusti, as well as vexillations of legions based on the German, Danubian and Syrian frontiers, totalling some 80-90,000 soldiers. This is what scholars call a `field army’, a modern term of convenience used to describe a large force composed of vexillations from a range of legions and auxiliary forces, which accompanied emperors or their leading generals on campaigns. Apart from the legio II Parthica, the only other legion that may have participated in Caracalla’s campaign as a complete unit was the legio II Adiutrix of Pannonia. This meant that the legio II Parthica was effectively the central core of the force and – although no ancient source explicitly attests this – the logical commander of the field army would be the praetorian prefect. Both of Caracalla’s prefects, M. Opellius Macrinus and M. Oclatinius Adventus, are known to have accompanied him to the east. This necessitated the appointment of a substitute prefect in Rome to handle the judicial responsibilities of the position.
The legio II Parthica later formed the core of the forces marshalled by Severus Alexander and Gordian III for their eastern campaigns against the revived Persian empire. Indeed, it is during Gordian III’s reign that the connection between the legion and the praetorian prefect is shown clearly for the first time. Both the emperor’s praetorian prefects, C. Furius Sabinius Aquila Timesitheus and C. Iulius Priscus, formed part of the retinue that left Rome for the Persian front in AD 242. In the same year, Valerius Valens, praefectus vigilum, is attested in Rome `acting in place of the praetorian prefect’ (vice praef(ecti) praet(orio) agentis). In this capacity he oversaw the discharge of the veteran soldiers of the legio II Parthica. These men had originally enlisted in AD 216, and had been left behind in Rome rather than journeying to the east. The prefects on campaign with their emperor became enormously powerful individuals: C. Iulius Philippus, who succeeded Timesitheus, was able to arrange the downfall of Gordian III in the east, and returned to Rome as emperor. Successianus, an equestrian commander on the Black Sea in the 250s, was summoned by Valerian to serve as his praetorian prefect in the east, where he commanded the field army against the Persians. The composition of Valerian’s army is strikingly demonstrated by the account of the Roman forces in the account of the Persian king Shapur, known as the Res Gestae divi Saporis. This includes the detail that the praetorian prefect was captured by the Persians in AD 260 alongside the emperor and members of the senate. The employment of the legio II Parthica as a permanent core of the emperor’s own field army enhanced and consolidated the position of the praetorian prefect as a senior military commander in addition to the senatorial generals.
The rise of the field armies attached to the emperor and the praetorian prefect sometimes offered new opportunities to soldiers of other ranks. In the previous section we observed the marked correspondence between soldiers who served in the praetorian guard, the equites singulares, and the legio II Parthica, and those who obtained advancement into the militiae equestres or the promotion of their sons to equestrian rank. Proximity to the emperor and his senior staff on campaign evidently had its advantages. The same phenomenon can be observed in the careers of prefects of the legio II Parthica, which, since it accompanied Caracalla to the east, was intimately bound up with the political machinations of the years AD 217-18. In this period the empire passed from Caracalla to his prefect Macrinus and then to the boy emperor Elagabalus, with the crucial battles all happening in Syria. The commanders of the legio II Parthica included Aelius Triccianus, who had begun his career as a rank-and-file soldier in Pannonia and ostiarius (`door-keeper’) to the governor. Other ostiarii are attested as being promoted to centurion, so it is likely that Triccianus himself became a centurion and primus pilus, a career path attested for comparable equestrian legionary prefects. This was a spectacular career, but not unprecedented or improper. The same can be said for P. Valerius Comazon, who served as a soldier in Thrace early in his career, before rising to become praefectus of the legio II Parthica. Again, there is nothing truly exceptional in and of itself about soldiers who ascended to the Rome tribunates or camp prefecture via the primipilate. But the command of the legio II Parthica offered connections to the imperial court, and the favour of Macrinus and Elagabalus, respectively, enabled Aelius Triccianus and Valerius Comazon to enter the ranks of the senate. Their promotion earned the ire of the senatorial historian Cassius Dio, who disliked the progression of soldiers into the amplissimus ordo. Dio did not resent the advancement of equestrians per se, but the elevation of soldiers who were able to enter the equestrian order and then into the curia. Triccianus and Comazon were quite different from M. Valerius Maximianus, who originated from the curial classes of Pannonia. Such opportunities would only become more common as emperors spent more time on campaign with their field armies.
In addition to the creation of the Parthian legions and the growing importance of the field army, the first half of the third century AD witnessed equites appointed to ad hoc procuratorial military commands. We have already noted this phenomenon in the wars of Marcus Aurelius, when M. Valerius Maximianus and L. Iulius Vehilius Gallus Iulianus commanded army detachments with the rank of a procurator, as a way of compensating for the lack of any defined military pathway for equestrians after the militiae. In the reign of Severus Alexander, P. Sallustius Sempronius Victor was granted the ius gladii with a special commission to clear the sea of pirates, a command that was probably associated with his existing procuratorship in Bithynia and Pontus. This creation of new military commands within the procuratorial hierarchy can also be seen vividly in the case of Ae[l]ius Fir[mus]. Following a series of financial procuratorships in Pontus and Bithynia and Hispania Citerior (high-ranking posts in and of themselves), Fir[mus] was placed in charge of vexillations of the praetorian fleet, detachments of a legio I (possibly Parthica or Adiutrix), and another group of vexillations, in the Parthian War of Gordian III. In this capacity he ranked as an army commander and procurator at the ducenarian level, without actually holding a standing military post (such as fleet prefect, praesidial procurator or praetorian prefect). The adaptability of the equestrian careers to meet the new demands is demonstrated by the case of a certain Ulpius [-].227After series of administrative procuratorial positions, Ulpius was praepositus of the legio VII Gemina. Since this legion was normally stationed in northern Spain, Ulpius probably commanded vexillations of the legion in a war conducted in the reign of Philip. He then returned to the usual procuratorial cursus, serving as sub- praefectus annonae in Rome.
Some equestrians were given special appointments as dux with responsibility for a specific province or series of provinces. This can be observed in Egypt, where generals with the title of dux or commander appear in the 230s-240s. The archaic Greek word σρατηλάτης is rarely used in the imperial period before the third century AD; the only exception is inscribed account of the career of the Trajanic senator and general C. Iulius Quadratus Bassus at Ephesus. But it makes a reappearance in the third century AD to describe senior equestrian military commanders. The first Egyptian example is M. Aurelius Zeno Ianuarius, who replaced the prefect in some, or probably all, of his functions in AD 231. His military responsibilities should be connected with the beginning of Severus Alexander’s Persian War. The second dux/σρατηλάτης mis attested ten years later, in AD 241/2, which is precisely when war broke out between Romans and Persians again under Gordian III. This time, the dux was Cn. Domitius Philippus, the praefectus vigilum, who appears to have been sent directly to Egypt while retaining his post as commander of the vigiles. In both cases the new military command was an ad hoc addition to their usual equestrian cursus. The final example occurs in the 250s, when M. Cornelius Octavianus, vir perfectissimus, is attested as `general across Africa, Numidia and Mauretania’ (duci per Africam Numidiam Mauretaniamque), with a commission to campaign against the Bavares. This substantial command was in succession to his appointment as governor of Mauretania Caesariensis. Octavianus then departed to become prefect of the fleet at Misenum, working his way to a senior post in the equestrian procuratorial cursus. All these cases show the essential adaptability of the imperial system, which allowed third-century emperors to appoint equestrians to senior military commands when it suited them. This may have been because an equestrian was the person the emperor trusted most in the circumstances; for example, Cn. Domitius Philippus, as praefectus vigilum, was one of the most senior officials in the empire. This represents the same pragmatic approach we saw in the appointment of equestrians as acting governors. On a practical level, it did not matter whether an army commander was an eques Romanus or a senator, because the military tasks that he was capable of performing, and was entrusted with by the emperor, were essentially the same. The new ad hoc army commands gave members of the equestris nobilitas further opportunities to serve the state domi militiaeque alongside the senatorial service elite.
At the same time, it is necessary to point out that these changes did not lead to senators being ousted from military commands prior to the reign of Gallienus. Rich epigraphic evidence, combined with the testimony of Dio and Herodian, preserves a long list of Septimius Severus’ senatorial generals. P. Cornelius Anullinus, L. Fabius Cilo, L. Marius Maximus, Ti. Claudius Candidus and L. Virius Lupus commanded Severus’ troops as duces or praepositi in one, or both, of his civil wars against Pescennius Niger and Clodius Albinus. Candidus also participated in the emperor’s Parthian campaigns, alongside Ti. Claudius Claudianus, T. Sextius Lateranus, Claudius Gallus, Iulius Laetus and a certain Probus. These senators were rewarded with a range of honours, from consulships and governorships to wealth and property (the sole exception was Laetus, who was executed for being too popular with the troops). In the face of such overwhelming testimony, it proves difficult to marshal support for the still-popular scholarly argument that Severus prioritised equestrian officers over senators. Equestrian commanders continued to participate in campaigns as subordinates to the senatorial generals, as we see in the case of L. Valerius Valerianus, who commanded the cavalry at the Battle of Issus under the authority of the consular legate, P. Cornelius Anullinus.
The same pattern can be found in Severus Alexander’s Persian War of AD 231-3. Herodian’s History, our major historical account of this conflict, is notoriously deficient in prosopographical detail. Yet senators are attested in inscriptions, as in the case of the senior consular comes, T. Clodius Aurelius Saturninus, who accompanied Alexander to the east. The senator L. Rutilius Pudens Crispinus, praetorian governor of Syria Phoenice and legate of the legio III Gallica, also served as a commander of vexillations during this conflict. But we only know about Crispinus’ command from an inscription from Palmyra, which recounts the assistance rendered by the local dignitary Iulius Aurelius Zenobius to Alexander, Crispinus and the Roman forces. The inscribed account of Crispinus’ career from Rome merely states that he was legatus Augusti pro praetore of Syria Phoenice. It is probable that senatorial governors, such as D. Simonius Proculus Iulianus, consular legate of Syria Coele, continued to play important roles in eastern conflicts under Gordian III. Indeed, the evidence for equestrian procurators acting vice praesidis in Syria Coele, discussed above, suggests that the procurator assumed judicial responsibilities while the consular governor was preoccupied with warfare. This indicates that senatorial governors continued to play a major part in military campaigns, even if it was not specifically noted in inscriptions recording their cursus.
This argument is supported by the literary sources that show senators assuming military commands through to the middle decades of the third century AD. We can observe this in particular in the Danubian and Balkan region, which was a near-continuous conflict zone. Tullius Menophilus fought against the Goths as legatus Augusti pro praetore of Moesia Inferior in the reign of Gordian III. During the incursion of the Goths under Cniva in AD 250/1, the Moesian governor C. Vibius Trebonianus Gallus successfully defended the town of Nova. In AD 253 M. Aemilius Aemilianus, governor of one of the Moesian provinces, pursued the fight against the Goths, before being acclaimed emperor. Senators also continued to receive special commands, as in the case of C. Messius Quintus Decius Valerinus and P. Licinius Valerianus, both future emperors, who were placed in charge of expeditionary forces by the emperors Philip and Aemilius Aemilianus, respectively. In Numidia, the governor C. Macrinius Decianus conducted a major campaign against several barbarian tribes in the middle of the 250s. In fact, if we examine the backgrounds of the generals who claimed the purple up to and including the reign of Gallienus, the majority of them were actually senators, a fact obscured by the common use of the term `soldier emperor’ for rulers of this period. Decius, one of the few known senators from Pannonia, successfully allied himself with an Etruscan senatorial family when he married the eminently suitable Herennia Cupressenia Etruscilla. His successor, Trebonianus Gallus, was of remarkably similar background to Etruscilla, coming from Perusia in central Italy. The emperor Valerian likewise had close links with the Italian senatorial aristocracy, marrying into the family of the Egnatii. Some of the more ephemeral emperors deserve notice too, such as Ti. Claudius Marinus Pacatianus, the descendant of a Severan senatorial governor, who rebelled in the reign of Philip. P. Cassius Regalianus, who was probably consular legate of Pannonia Superior when he began an insurrection against Gallienus in 260, was himself descended from a Severan suffect consul. These men were not soldiers promoted from the ranks, but senatorial generals who used their positions to make a play for the imperial purple.
The Roman military hierarchy in the first half of the third century AD was therefore characterised by a mixture of continuity and change. The creation of the legio II Parthica, and the necessity for the emperor and his praetorian prefects to campaign on a regular basis, meant that emperor was in close contact with members of the expeditionary forces. Officers in the field army could receive imperial favour and embark on spectacular careers, like Aelius Triccianus or Valerius Comazon, or even Iulius Philippus, the praetorian prefect who snatched the purple from Gordian III while in the east. It is no coincidence that many of the soldiers’ sons attested with equestrian rank belonged to the praetorian guard, the equites singulares and the legio II Parthica. At the same time, the imperial state tried to create senior army roles for promising equites in a manner analogous to senatorial legates by instituting ad hoc procuratorial commands (as seen in the case of Valerius Maximianus and Vehilius Gallus Iulianus). This gave members of the equestris nobilitas, the equestrian aristocracy of service, access to army officer commands beyond the militiae equestres. It should be noted that for the most part these men were not lowborn ingénues from the ranks, but members of the municipal aristocracy who served the res publica in a comparable manner to senators, as their predecessors had before them. It is also imperative to point out the endurance of tradition within the high command. Senatorial legates and generals still commanded armies in the emperor’s foreign wars on the Rhine, Danube and Euphrates frontiers. Their military authority continued to make them viable and desirable candidates for the purple in the first half of the third century AD. There was as yet no attempt to undermine the positions of senatorial tribunes or legionary legates. It was the dramatic developments in the 250s-260s that provided the catalyst to set the empire on a radically different path.