A paper by Lucas McMahon, for 2015’s Islam seminar organized by Michael Cook.
With the arrival of the Romans to the eastern Mediterranean, a system of two empires emerged. The Romans dominated Asia Minor, Syria, Egypt, and parts of northern Mesopotamia and Armenia, while the Persians (first under the Parthian Arsakid dynasty and from the third century, the Sasanian dynasty) controlled most of the Tigris and Euphrates valleys and Iran across the Zagros Mountains. Although the history of Rome and Persia is replete with warfare, these wars are notable for changing very little in the grand strategic outlook of the near east. This system persisted because it proved a benefit to both sides. This system failed to persist after the first decades of the seventh century because despite efforts to prop it up, the increasingly expansive, expensive, and acrimonious attitude towards warfare between Rome and Persia took what had previously been a series of border conflicts to a much higher level, ultimately destabilizing both states. Both Rome and Persia had undergone periods of serious tribulation before and survived, but this time the emergence of the new Islamic polity on a previously untroubled frontier at a time of political weakness shattered the hegemony of both states to the extent that they were unable to recover.
The Romans and Persians benefited from each other’s presence in the near east, which led to a system that would eventually refer to the Roman emperor and Persian king of kings as the “Two Eyes of the Earth”, thus giving them both a stake in maintaining the world order. Although somewhat over-generalized here, fighting between Romans and Persians was a good way to secure prestige and wealth for rulers and to help them keep their thrones. In general this held true through much of the period and is particularly notable in the third century, where instability in the Roman Empire and the birth of the Sasanian dynasty led both sides to seek prestige and security in military victory. The failed campaigns of the republic led by Crassus and Antony can be seen in this light as well, since both were in active competition with Roman rivals active in other theatres. During the fourth century, both Roman and Persian kingship came to adopt stronger claims to divinity, and despite both rulers using exclusivist language that made set each of them up as sole rulers of the world, the shared vocabulary allowed a level of communication and tolerance. After the defeat and death of the emperor Julian on a Mesopotamian campaign in A.D. 363, Sasanian Persia gained the strategic city of Nisibis, which altered the frontier in such a way as to make any Roman advance in Mesopotamia difficult. A long peace punctuated with minor flare-ups of hostility persisted until the end of the fifth century, largely aided by both empires having their attention turned towards the steppe. Scenes of violence in art and rhetoric in this period did not necessarily manifest into conflict, and both rulers were able to benefit by still receiving prestige and yet focusing their energies on other problems. Cooperation between the two empires even took place. Throughout the Roman occupation of northern Mesopotamia, the borders remained largely the same, and major successes usually resulted in only minor shifts in the frontier.
The sixth century saw the expansion of Romano-Persian warfare beyond Mesopotamia. In 502 the Persian king Kavadh started a war, probably with the intention of replenishing the Persian treasury and gaining some military authority since he owed his throne to the Hepthalite Huns. Kavadh could hope for a short, profitable war given that the two fifth-century conflicts were resolved within the year and led to the payment of Roman subsidies. He was wrong, and the Romans chose to fight. Stalemate and long-term hostility resulted. The Romans also engaged in a serious fortification program. This required that if either side wanted to move the frontier in Mesopotamia, significant resources and time would have be devoted to sieges. The result was that campaigns increasingly took place elsewhere, notably in Armenia and Lazika (although a Persian attempt to outflank the Romans in southern Arabia was also undertaken), and war in Mesopotamia was focused more on the Arab tribesmen employed by both sides and large-scale raids. The ill-named Eternal Peace of 532 was soon broken, leading to another two decades of conflict. A decade of unsettled peace (562-572) ended with Justin II’s invasion of Mesopotamia, a conflict which lasted into the early 590s. The established peace was unstable, with the Persian king Khusro II placed on the throne by the emperor Maurikios and the king was soon forced to deal with a revolt, while Roman forces were deployed to the Balkans. That Maurikios could shelter and eventually install a member of the House of Sasan reveals that even after a century of hostility the two empires desired the geopolitical system to continue. The installation of Khusro also reveals that both were taking the eastern wars more seriously than before and pushing their opponents harder in them. Peace did not necessarily mean peace, as each side sought to outmaneuver the other even during times when war was not declared.
The result of this was instability. When Khusro declared war on Rome in 602, it seems quite unlikely that he intended to expand his war as much as he would, and probably just searching for a reversal of the 591 treaty that had cost Persia strategic depth. The decision to liquidate the Roman state probably came only after a significant victory over Herakleios in Syria and that emperor’s groveling attempt to end the war. The Roman response was both a clever and daring assault into Mesopotamia in late 627, drawing on tension between Khusro II and his supporters. The fall of Khusro’s regime did not see the Romans attempting to push their advantage. Rather, they were content to allow that empire that had nearly just destroyed them to continue to exist. However, the political unrest that emerged in both states allowed for the Arabs, previously not considered very threatening but who had undergone a degree of centralization and enrichment as clients over the previous two centuries to take advantage of the situation.
The AD 250s were a disastrous decade for the Romans, with the emperor Decius killed fighting along the northern frontier in 251 and the emperor Valerian captured by the Persians in 260. Out of this political chaos, the city of Palmyra emerged as an effectively independent state, its militia armies able to defeat the Persian king Shapur and conquer parts of the Roman near east. Prior to the seventh century, this was the single greatest deviation from the stable two-empire system. The difference in the third century is that the political situation for the Romans was more fortuitous. In the late 260s, the Romans won significant victories over the “Goths” in the Balkans. The reigning emperor Claudius II Gothicus died and the army raised up Aurelian, a Balkan-based military commander. Aurelian’s rival Quintillus was soon dead, and Aurelian pursued a Germanic army into Italy, securing his control over the peninsula in the process. Returning to the Balkans, Aurelian won another major victory over the “Goths” and shortened up the Danubian frontier. The result was that Aurelian had the prestige to keep him safe from his own soldiers and had a significant power base with which to launch a major campaign against Palmyra in the near east.
This strategic situation was not shared by the Byzantine state in the seventh century. In AD 641, Constans II came to the throne at the age of 14. Internal strife brought him to the throne, and his early years witnessed a rebellion in Africa, Arab invasions, and the domination of the military by the patrikios Valentinian. Constans’s regime was shaken by defeat, but despite this he appears to have followed a strategy similar to that of Aurelian by attempting to secure western strategic depth, and a strategy similar to his grandfather Herakleios by finding allies in the Caucasus. Restoring some Byzantine sovereignty over the Balkans also seems to have been a goal of Constans’s regime, and an attempt was made on Egypt. Neither Herakleios nor Constans were ever able to reacquire the strategic depth and resources that Aurelian had been able to muster when he re-conquered the Roman east from Palmyra. Whereas Aurelian possessed much of the central Mediterranean, Constans had only an embattled Africa and Italy, indeterminate sections of the Balkans, a besieged Asia Minor, and evidently lacked the resources or confidence to take full advantage of the First Fitna. These actions of Constans’s government are important since they reveal an attempt to return Constantinople to its former hegemony over the eastern Mediterranean. The earlier introduction of a third power to the region had occurred under similar circumstances, but in that case sufficient resources could be mustered to return to the status quo. The new invaders were as proficient at warfare as those they had supplanted, and they struck at a time when the formerly vast resources of the two empires could not be brought against them. The stable two-empire situation did not last longer because it was broken at a moment of political weakness, and despite attempts to re-assert it, that break was too severe and too sudden for either power to recover.
 Matthew Canepa, The Two Eyes of the Earth: Art and Ritual of Kingship between Rome and Sasanian Iran (Los Angeles: University of California Press, 2009), 1-5.
 J. W. Drijvers, “Rome and the Sasanid Empire: Confrontation and Coexistence,” in ACompanion to Late Antiquity, ed. Philip Rousseau (Hoboken, NJ: Wiley-Blackwell, 2009), 446 sees three periods: Sassanian aggression (226-363, coexistence (363-500), and increasing hostility (500-630). The label assigned to this first period can be critiqued as it saw just as much Roman aggression as Persian, with no less than Alexander Severus, Gordian III, Valerian, Carus, Galerius, Constantine, Constantius II, and Julian all leading campaigns into Persian territory (and Constantine I planning a campaign), with several going as far as Ctesiphon.
 Warwick Ball, Rome in the East: The Transformation of an Empire (London: Routledge, 2000), 107-110. B.A. Marshall, Crassus: A Political Biography (Amsterdam: Adolf M. Hakkert, 1976), 144-9.
 Canepa, Two Eyes, 100-6.
 Geoffrey Greatrex, Rome and Persia at War, 502-532 (Leeds: Francis Cairns, 1998), 11-15.
 Geoffrey Greatrex, “Byzantium and the East in the Sixth Century,” in The Cambridge Companion to the Age of Justinian, ed. Michael Maas (Cambridge: Cambridge University Press, 2005), 480-6. R. Malcolm Errington, Roman Imperial Policy from Julian to Theodosius (Chapel Hill: University of North Carolina Press, 2006), 66-7.
 Canepa, Two Eyes, 122-154.
 Geoffrey Greatrex and Samuel N.C. Lieu, The Roman Eastern Frontier and the Persian Wars Part II: AD 363-630 (London: Routledge), 33-4, 56-8.
 See Geoffrey Greatrex, “Roman Frontiers and Foreign Policy in the East,” in Aspects of the Roman East I: Papers in Honour of Professor Fergus Millar FBA, ed. Samuel N.C. Lieu and Richard Alston (Turnhout: Brepols, 2007), 118-123 for the argument that the Romans adopted sensible limits to their empire, recognizing the difficulty in communications and control by going much further. Cf. Garth Fowden, Empire to Commonwealth: Consequences of Monotheism in Late Antiquity (Princeton: Princeton University Press, 1993), 17-19 who sees the northern Mesopotamian frontier as a stalemate. A good example of the willingness to maintain two empires is AD 363, where despite a crushing Roman defeat and the death of the emperor Julian in battle, the Persians were content with taking the single critical frontier city of Nisibis, and even allowed the Roman army to return home.
 Greatrex, Rome and Persia at War, 5-8.
 Greatrex and Lieu, Eastern Frontier, 74-8.
 Greatrex and Lieu, Eastern Frontier, 102-18.
 Maurikios was willing to remove a commander from Dara whom Khusro disliked, suggesting some amicability: Theophylact Simocatta (trans. Whitby) 8.15.2. Cultural understanding was evidently sufficiently advanced that Khusro could create a convincing pretender of Maurikios’s son Theodosios who managed to secure the surrender of Theodosioupolis: Canepa, Two Eyes, 187; Walter Kaegi, Heraclius, Emperor of Byzantium (Cambridge: Cambridge University Press, 2005), 67-8.
 Greatrex, Rome and Persia at War, 120-135.
 Irfan Shahîd, “The Last Sasanid-Byzantine Conflict in the Seventh century: the Causes of its Outbreak,” in La Persia e Bisanzio , ed. Antonio Carile (Rome: Accademia Nazionale dei Lincei, 2004), 227-8, 234-5.
 James Howard-Johnston, “Heraclius’ Persian Campaigns and the Revival of the East Roman Empire,” War in History 6 (1999), 1-3. Kaegi, Heraclius, 65. Cf. Josef Wiesehöfer, “The Late Sasanian Near East,” in The New Cambridge History of Islam: The Formation of the Islamic World, ed. Chase Robinson (Cambridge: Cambridge University Press, 2010), 113, 138. Note also the post-614 attempt to manage affairs in Jerusalem, indicating that the Sasanians were ruling Syria, rather than just pillaging it: Yuri Stoyanov, Defenders and Enemies of the True Cross: The Sasanian Conquest of Jerusalem in 614 and Byzantine Ideology of Anti-Persian Warfare (Vienna: Österreichische Akademie der Wissenshaften, 2010), 46-53.
 James Howard-Johnson, “Pride and Fall: Khusro II and His Regime, 626-628,” in La Persia e Bisanzio, ed. Antonio Carile (Rome: Accademia Nazionale dei Lincei, 2004), 93-113. Parvaneh Pourshariati, The Decline and Fall of the Sasanian Empire: The Sasanian-Parthian Confederacy and the Arab Conquest of Iran (London: I.B. Tauris, 2008), 153-60.
 Even T’ang China attempted to intervene by establishing a Persian protectorate, but this was quickly conquered by the Arabs: Robert Hoyland, In God’s Path: The Arab Conquests and the Creation of an Islamic Empire (Oxford: Oxford University Press, 2015), 87.
 Greg Fisher, Between Empires: Arabs, Romans, and Sasanians in Late Antiquity (Oxford: Oxford University Press, 2011), 79-80, 93-6, 117-20. the Romans never prepared to fight the Arabs. The Strategikon, a late sixth or early-seventh century military manual attributed to the emperor Maurikios includes sections on fighting the Persians, steppe peoples, Germanic peoples, and some of the more recent arrivals to the Balkans, but nothing on Arabs: Maurice, Das Strategikon des Maurikios, ed. George T. Dennis (Vienna: Österreichische Akademie der Wissenshaften, 1981), XI. Roman thought outside of the military manual believed that Arabs were unable to attack fortifications, and that the best way to fight Arabs was with other Arabs, further disarming the Roman state from a southern threat: Walter Kaegi, Byzantium and the Early Islamic Conquests (Cambridge: Cambridge University Press, 1992), 56-7.
 David Potter, The Roman Empire at Bay AD 180-395 (London: Routledge, 2004), 244-6, 255-6.
 Potter, Empire at Bay, 251, 259-61, 266-72.
 Alaric Watson, Aurelian and the Third Century (London: Routledge, 1999), 39-54.
 Watson, Aurelian, 54-6. Potter, Empire at Bay, 268-70.
 Peter Sarris, Empires of Faith: The Fall of Rome to the Rise of Islam, 500-700 (Oxford: Oxford University Press, 2013), 279-82. cf. Cyril Mango and Roger Scott, The Chronicle of Theophanes Confessor: Byzantine and Near Eastern History AD 284-813 (Oxford: Clarendon Press, 1997), 476, n. 1 who claim Constans was ten years old at the time of his accession.
 Theophanes Confessor, Chronographia, ed. Charles De Boor (Leipzig: B.G. Teubner, 1883), 342-345.
 In general, Sarris, Empires of Faith, 287-291. James Howard-Johnston, Witnesses to a World Crisis: Historians and Histories of the Middle East in the Seventh Century (Oxford: Oxford University Press, 2010), 479-87. For the journey to Italy: Constantin Zuckerman, “Learning from the Enemy and More. Studies in “Dark Centuries” Byzantium,” Millennium: Jahrbuch zu Kultur und Geschichte des ersten Jahrtausends n. Chr 2 (2005), 79-104. Salvatore Cosentino, “Constans II and the Byzantine Navy,” Byzantinische Zeitschrift 100.2 (2008), 594-603.
 Marek Jankowiak, “The First Arab Siege of Constantinople,” Travaux et Mémoires 17, 305 accepts Agapios’s claim that Constans used his new navy in Sicily for carrying out attacks on Slavic settlements in the Balkans. Presumably the march and wintering in Athens was intended to shore up Byzantine power in the south Balkans as well: Liber Pontificalis (ed. Duchesne), 343. Egypt: Hoyland, God’s Path, 76.