The question of the Roman-Byzantine desert frontier in Late Antiquity has attracted the attention of various scholars in the past several decades. This frontier, known as the Limes Arabicus, spanned more than 1,300 kilometers from northern Syria to southern Palestine and the edges of the Arabian Peninsula. Recent research and excavations have done much to augment scholarly knowledge of this frontier, which served as the south-eastern defense of the Roman Empire for over six centuries, until it was eventually overrun by the Arab Muslim conquerors in the 630s. This frontier consisted not only—nor even primarily—of fortifications and watchtowers, but also of major nomadic tribal confederations, allied to Rome, which patrolled the desert and ensured the security of the eastern provinces by exercising control over nomadic movements both within and, to a certain degree, beyond the boundaries of the Roman Empire. Although much attention in recent years has been devoted to the Ghassānids, which served as a major tribal confederation allied to Rome well into the seventh century, the fourth century has been relatively less discussed. Those scholars, including Irfan Shahid, Greg Fisher and Glen Bowerstock, who have devoted their scholarship to understanding this period have indicated its importance and have sought to underscore that the fourth century witnessed both the rise of major Arab tribal confederations along the desert frontier and the establishment of Roman foedus agreements, or alliances, with them.
As a critical presentation of modern historiography and a close reading of the primary literary sources shows, three major trends emerge when considering the relationship between Rome and the Arab tribes in the fourth century: 1) the military and political significance of the Arab tribal confederations as a factor on Rome’s eastern frontier, 2) the increasing Christianization of the Arab tribes of Sinai and Syria during this period as a result of the activities of ascetics and monks dwelling in the desert, and 3) the increasing military, administrative, and cultural integration of the Arabs into the Roman-Byzantine system in the Limes Arabicus. These three developments became increasingly interlinked during the late fourth century as Christianization became an important tool for the integration of Arab tribes on the frontier into the Roman-Byzantine sphere of influence.
As Philip Mayerson has argued, the frontier was not merely a “border” separating Romans from nomads, but rather a major zone of contact which included both defensive fortifications and thinly-populated settlements between Palestine and the Euphrates River, thereby encompassing large areas within the Roman provinces of Egypt, Palestine, Syria, and Arabia. Indeed, even the relationship between the nomadic Arabs and the settled population (both the local population and the Roman authorities) was quite complex, which ranged from symbiotic coexistence to violent raiding and open warfare. The desert frontier should not, therefore, be thought of as a barrier of separation between “Romans” and “Arabs,” or between pastoralists and settled peoples but as a zone which offered the opportunity of peaceful interchange, and one which was occasionally subjected violent incursions from groups of nomadic tribesmen in search of loot and slaves. Several scholars, including Mayerson, have highlighted how the system of border fortifications and watchtowers along the Limes Arabicus was strengthened during the era of the Emperor Diocletian (r. 284–305), who reorganized the desert frontier and reinforced its defensive infrastructure.
As important as this “line of defense”, extending roughly from Damascus to southern Palestine, may have been, scholars have emphasized how this nevertheless did not protect the eastern provinces from the raids and depredations of the nomadic desert tribes. Rather, many of these fortresses and watchtowers served as an alert system and a means to provide safe refuge for the inhabitants of these regions when such raids did in fact occur. It is crucial to keep in mind that Arab raids against the population centers of the eastern provinces were often immensely destructive to property and often claimed a large number of people, who were either killed or carried off into slavery, which led to the depopulation of significant regions in the provinces of Syria and Palestine. These raids were carried out by both Sassanid-allied Arabs, namely the Lakhmid/Nasrid confederation, as well as more autonomous bands of raiders who dwelled along the Roman-Byzantine desert frontier, and were not aimed at occupying or holding territory, but at securing wealth (both loot and slaves).
It was incredibly difficult for the Roman soldiers stationed along the desert frontiers to adequately prevent raids by the nomads, especially since many of these tribesmen resided within Roman imperial territory, which led to the adoption of a particular strategy in order to reinforce the defenses in the east.  A key component of this strategy was, as already mentioned, the infrastructure of watch-towers and fortresses which served to secure the lines of communication, monitor the movement of the tribes and serve as places of refuge for the rural population of the province, rather than as a comprehensive attempt to prevent Arab raids altogether, something which was impossible given the extent of the frontier and the nature of the desert. Mayerson underscores that from the outset, the Romans recognized the futility of securing the desert frontier from nomadic raids, and adapted accordingly. As such, at least as early as the fourth century, the Romans began a process of integrating several Arab tribes from northern Arabia and Syria into their defensive system by entering into agreements (foedus) with important tribal chiefs who would ensure that order was maintained among those tribes under his (or, in some cases, her) authority while preventing raids by those hostile nomads beyond the Roman frontier.
In this way, the Roman-allied nomadic tribes could serve as a buffer (or, “defensive shield”, as some have termed it) between the eastern provinces and the hostile nomads beyond imperial authority. These semi-nomadic tribes were beneficial to the Romans because of their fighting skills, well-adapted to desert warfare, as well as their mobility as light cavalry. The title usually conferred upon Arab chieftains operating under the auspices of Roman authority as foederati was “phylarch” (φύλαρχος). There is much that remains unclear about the exact administrative function or the authority of the phylarchs, but it appears that they were powerful or important enough that they usually exercised a certain degree of control over nomadic tribes both within Roman territory and across the desert frontier. The famous Namāra inscription (328 A.D.), highlighted and studied by several scholars seems to provide an early indication of this arrangement of Arab foederati operating along the desert frontier. As the inscription suggests, the chieftain in question (Imru’ al-Qays) was the head of a nomadic confederation acting on behalf of Rome on the desert frontier, whose activities included (preventive?) warfare against the nomadic Arabs beyond the boundaries of the Roman Empire.
It is significant that several scholars have emphasized that he had defected to Rome after having served as an ally of the Sassanids, highlighting the gradual gravitation of certain Arab tribes into the Roman sphere of influence. Apparently the influence of Imru’ al-Qays was such that he could claim to be the “king of all the Arabs”, which suggests that although he was a phylarch, bound by treaty with Rome, he also exercised a form of independent authority in his own right over a large number of nomadic tribes. The foederati pact served not only a direct military-defensive purpose but also brought many Arab tribes within the cultural and political orbit of Rome, a fact made even more apparent by the adoption of Christianity by several Arab tribes.
A major trend emphasized by modern scholarship has been the rise of powerful, nomadic (Arab) confederations along the desert frontier in the fourth century. Following the collapse of the Nabataean and Palmyrene kingdoms, which witnessed the emergence of certain groups of Arabs as an independent political force in the Near East during the second and third centuries, there was a brief restoration of Roman authority along the Syro-Arabian frontier. However, by the early to mid-fourth century, as evidenced in part by the Namāra inscription, there appears to have been a rise of new tribal confederations which filled the power vacuum caused by the fall of the Palmyrene kingdom.
Although it is difficult to ascertain with any certainty, it has been argued that one of these tribal confederations was that of the Tanūkhids which arose in the late third century and had entered into an alliance with Rome by the early fourth century. There have been suggestions that the rise of these tribal confederations along Rome’s desert frontier were caused by a number of factors, including the relative neglect of the south-eastern provinces of the empire due to the preoccupation of imperial forces elsewhere in the Empire. Regardless of the causes which facilitated the rise of these new tribal confederations, which represented regional powers and posed a major strategic and military threat to the eastern provinces, which were subjected to devastating raids and whose populations were often carried off into slavery, once they did arise the Roman provincial and imperial authorities were resolved to deal with them. The rise of the Arab tribal confederations was a factor which should be seen alongside the already-existing challenge of Persia and the threat posed by its Arab allies. Thus, as Mayerson explains, “circumstances compelled the Romans to use other Saracens to counter those whom Persia employed. It is doubtful that they would have entered into an alliance with a coalition of Arab tribes if they had had another option.”
It was in the face of these geo-political and strategic challenges that the foederati arrangement became an important component of Roman defensive strategy as a means of both pacifying these tribes and ensuring the security of the frontier by “subcontracting” the defense of the Limes Arabicus to these nomadic confederations, which removed much of the pressure from the Roman civil and military authorities while also relieving the sedentary population from the major raids carried out by Persian-allied Arab tribes. The relationship between Arab nomadic tribesmen and Rome was constantly in flux, and it was complicated by a number of factors. Primarily, it was unlikely that the tribal chieftain, or phylarch, could always restrain the tribes under his authority from engaging in raids against the sedentary population, thereby undermining the effectiveness of this “defensive shield” on Rome’s frontier. Moreover, there was also the possibility that the phylarch himself could break his allegiance with Rome for a variety of reasons, including dissatisfaction with the authorities, which would leave the eastern provinces exposed to major devastation, often by the very forces which were hitherto foederati on Rome’s behalf.
It is this fluidity of the relationship between Rome and its nomadic foederati, whereby the Arab tribes could be on good terms with the Romans at one point, while becoming their worst enemies the next which lay at the heart of Ammianus’ statement about how the Saracens “were desirable neither as friends nor foes.” It is also possible that this inability of Rome to secure the good-will of the desert tribes, even those ostensibly allied with it, which may have contributed to the death of the Emperor Julian in 363, although the specific circumstances remain unclear. For their part, the nomadic tribesmen benefited from the foederati arrangement by being granted material rewards (food and money), as well as being extended noble dignities and titles which undoubtedly enhanced their own standing in the eyes of their fellow nomadic Arabs. Overall, it seems to be the case that many scholars agree that the Arab confederations and tribesmen constituted a semi-autonomous force, “neither entirely outside nor inside the sphere of Roman authority.”
Another question which has been raised by scholars relates to the degree of control exercised by Rome over the nomadic Arab tribes allied to them through the foederati arrangement. One key point of contention has been whether the term “phylarch” should be interpreted as an official Roman administrative title or merely an honorary designating the chieftain to whom Rome was principally allied. In this regard, it is important to highlight that there was a distinction which was drawn between those Arabs (or “Saracens”) who served as units within the Roman military and those who served as foederati on the desert frontier. As the Notitia Dignitatum indicates, there were several active units of Saracen cavalry (favored for their mobility and military skills) within the Roman military, stationed mainly in the provinces of Egypt, Arabia, Mesopotamia, and Phoenicia around the year 400. These were distinct (administratively and, possibly, culturally) from the major Arab tribal confederations which were subsumed under the category of foederati and were under the direct authority of the phylarch. Nevertheless, those units fighting within the Roman army as well as the foederati all participated in the campaigns of the Empire on the eastern frontier against the Persians, with the foederati playing a more active role in counter-balancing the Persian-allied nomadic confederation of the Lakhmids.
The distinction between these two categories of Arab troops in imperial service serves to highlight an additional point, namely that the frontier zone in the east was home to various populations of Arabs. In the fourth century, populations of “Saracens” (the term employed by the literary sources) were widely spread across the frontier and, for many, their homeland was located within the very heart of the Roman provinces. These communities were extremely varied and included sedentary as well as nomadic groups, urban-based as well as rural inhabitants, and pagans as well as Christians. In other words, it is quite important to keep in mind that the term “Arab” or “Saracen” does not refer exclusively to the nomadic confederations, but encompasses a variety of populations within the eastern provinces of Syria, Phoenicia, Arabia, and Mesopotamia. However, to be sure, it was usually in their capacity as hostile raiders or allied foederati that most of the literary sources concern themselves with the “Saracens”; the sedentary Arab population of Syria seldom attracted the interest of the chroniclers.
 The pioneering work of Irfan Shahid, especially his multi-volume Byzantium and the Arabs in the Sixth Century (Washington D.C.: Dumbarton Oaks Research Library and Collection, 1995), in particular, has done much to forward scholarly understanding of the Ghassānids. Another recent work, namely Greg Fisher’s Between Empires: Arabs, Romans, and Sasanians in Late Antiquity (Oxford: Oxford University Press, 2011), has provided a further contribution to the study of the Arabs and the late Roman Empire in the sixth and seventh centuries, especially with reference to the Ghassānids.
 Philip Mayerson, “Saracens and Romans: Micro-Macro Relationships,” Bulletin of the American Schools of Oriental Research, No. 274 (1989), p. 71.
 Philip Mayerson, “The Saracens and the Limes,” in Monks, Martyrs, Soldiers, and Saracens: Papers on the Near East in Late Antiquity (Jerusalem: Israel Exploration Society, 1994), p. 39.
 S. Thomas Parker, Romans and Saracens: A History of the Arabian Frontier (Winona Lake, IN: Eisenbrauns, 1986), p. 9; Mayerson, “The Saracens and the Limes,” p. 39; Mayerson, “Saracens and Romans,” p. 75.
 Noel Lenski, “Captivity and Slavery among the Saracens in Late Antiquity (ca. 250–630 C.E.),” AnTard 19 (2011), pp. 244–249; Mayerson, “Saracens and Romans,” p. 73
 Mayerson, “Saracens and Romans,” p. 72; Parker, Romans and Saracens, p. 144
 Mayerson, “The Saracens and the Limes,” pp. 36, 39; Mayerson, “Saracens and Romans,” p. 75.
 Mayerson, “The Saracens and the Limes,” p. 39; Noel Lenski, “Captivity and Slavery among the Saracens in Late Antiquity (ca. 250–630 C.E.),” AnTard 19 (2011), pp. 255–256.
 Warwick Ball. Rome in the East: Transformation of an Empire (London: Routledge, 2000), p. 96; Irfan Shahid, “Byzantium and the Arabs during the Reign of Constantine: The Namāra Inscription, An Arabic Monumentum Ancyranum, A.D. 328,” Byzantinische Forschungen 26 (2000), p. 78; Greg Fisher, “A New Perspective on Rome’s Desert Frontier,” Bulletin of the American Schools of Oriental Research (2004), p. 54; Mayerson, “The Saracens and the Limes,” pp. 35–36, 43–44; Mayerson, “Saracens and Romans,” p. 76
 Mayerson, “Saracens and Romans,” p. 76
 Ball. Rome in the East, p. 96; Mayerson, “Saracens and Romans,” p. 76; Fisher, “A New Perspective on Rome’s Desert Frontier,” p. 55
 Ball. Rome in the East, p. 96; Mayerson, “The Saracens and the Limes,” p. 43; Philip Mayerson, “The Use of the Term “Phylarchos” in the Roman-Byzantine East,” Zeitschrift für Papyrologie und Epigraphik (1991), p. 291; Lenski, “Captivity and Slavery among the Saracens,” p. 241
 For a detailed discussion of the term “phylarch”, see Mayerson, “The Use of the Term “Phylarchos,” pp. 291–295
 Irfan Shahid, Byzantium and the Arabs in the Fourth Century (Washington D.C.: Dumbarton Oaks Research Library and Collection, 1984), pp. 31–52; J. Spencer Trimingham, Christianity among the Arabs in Pre-Islamic Times (Beirut: Librairie du Liban, 1990), p. 93; Ariel S. Lewin, “Amr ibn ‘Adi, Mavia, the Phylarchs and the Late Roman Army: Peace and War in the Near East,” in The Late Roman Army in the Near East from Diocletian to the Arab Conquest: Proceedings of a Colloquium held at Potenza, Acerenza and Matera, Italy (Oxford: Archaeopress, 2007), eds. Ariel S. Lewin and Pietrina Pellegrini, p. 245; Ball. Rome in the East, pp. 97–98; Lenski, “Captivity and Slavery among the Saracens,” p. 242; Shahid, “Byzantium and the Arabs during the Reign of Constantine,” p. 110.
 Lenski, “Captivity and Slavery among the Saracens,” p. 242
 Noel Lenski, Failure of Empire: Valens and the Roman State in the Fourth Century A.D. (Berkeley: University of California Press, 2002), p. 201; Shahid, “Byzantium and the Arabs during the Reign of Constantine,” pp. 88–91; Lewin, “Amr ibn ‘Adi, Mavia, the Phylarchs and the Late Roman Army,” p. 245; Shahid, Byzantium and the Arabs in the Fourth Century, p. 52.
 Ball. Rome in the East, p. 98; Lenski, “Captivity and Slavery among the Saracens,” p. 242; Lenski, Failure of Empire, p. 202; Lewin, “Amr ibn ‘Adi, Mavia, the Phylarchs and the Late Roman Army,” p. 245.
 Lenski, “Captivity and Slavery among the Saracens,” pp. 239–240, 242
 Ball. Rome in the East, p. 97
 Ball. Rome in the East, p. 97; Lenski, Failure of Empire, p. 202. Ball argues that Imru’ al-Qays, like Mavia, also belonged to the powerful Tanūkhid tribal confederation which arose on the northern fringes of the Arabian Peninsula in the fourth century.
 Ball. Rome in the East, p. 97
 Lenski, Failure of Empire, p. 202
 Mayerson, “Saracens and Romans,” p. 77
 Mayerson, “Saracens and Romans,” p. 76; Mayerson, “The Saracens and the Limes,” p. 43
 Lenski, “Captivity and Slavery among the Saracens,” p. 257
 See, for example, the case of al-Nu’man in Mayerson, “The Use of the Term “Phylarchos”, p. 293 n. 11
 Ammianus Marcellinus, History (Cambridge, Massachussets: Harvard University Press, 1958), trans. John C. Rolfe, 14.4
 Lenski, Failure of Empire, p. 203; Jan Retsö, The Arabs in Antiquity: Their History from the Assyrians to the Umayyads (London: RoutledgeCurzon, 2003), p. 514
 Mayerson, “Saracens and Romans,” p. 77; Mayerson, “The Saracens and the Limes,” p. 44; Lenski, “Captivity and Slavery among the Saracens,” p. 264
 Lenski, Failure of Empire, p. 203
 Mayerson, “The Use of the Term “Phylarchos”, p. 294
Robert G. Hoyland, “Arab Kings, Arab Tribes and the Beginnings of Arab Historical Memory in Late Roman Epigraphy,” in From Hellenism to Islam: Cultural and Linguistic Change in the Roman Near East (Cambridge: Cambridge University Press, 2009), ed. Hannah M. Cotton at al., p. 381; Mayerson, “The Use of the Term “Phylarchos,” p. 291–292
 David Woods, “The Saracen Defenders of Constantinople in 378” Greek, Roman, and Byzantine Studies 37: (1996), p. 271; Retsö, The Arabs in Antiquity, p. 511; Mayerson, “Saracens and Romans,” p. 76
 Mayerson, “The Saracens and the Limes,” p. 43
 Mayerson, “The Saracens and the Limes,” p. 36