While the conflict between the two civilizations did involve direct military engagements, a significant role was played by a plethora of vassal kingdoms and allied nomadic nations, which served as buffer states or proxies for either side.

Despite nearly seven centuries of hostility, the Roman–Persian Wars had an entirely inconclusive outcome, as both the Byzantines and the Sasanians were attacked by the Rashidun Caliphate as part of the early Muslim conquests, which had begun under Muhammad a few years earlier in the adjacent Arabian Peninsula.

[2] Ultimately, the expense of resources during the Roman–Persian Wars proved catastrophic for both sides, as the prolonged and escalating warfare of the 6th and 7th centuries left them militarily exhausted and vulnerable in the face of the sudden emergence and expansion of the Rashidun army.

The Parthians raided Syria the following year, and mounted a major invasion in 51 BC, but their army was caught in an ambush near Antigonea by the Romans, and they were driven back.

[14] With Roman control of Syria and Judaea restored, Mark Antony led a huge army into Atropatene, but his siege train and its escort were isolated and wiped out, while his Armenian allies deserted.

Trajan subdued the rebels in Mesopotamia, but having installed the Parthian prince Parthamaspates on the throne as a client ruler, he withdrew his armies and returned to Syria.

[21] Trajan's Parthian War initiated a "shift of emphasis in the 'grand strategy of the Roman empire' ", but his successor, Hadrian, decided that it was in Rome's interest to re-establish the Euphrates as the limit of its direct control.

In 195–197, a Roman offensive under the Emperor Septimius Severus led to Rome's acquisition of northern Mesopotamia as far as the areas around Nisibis, Singara and the third sacking of Ctesiphon.

[28] The struggle resumed and intensified under Ardashir's successor Shapur I; he invaded Mesopotamia and captured Hatra, a buffer state which had recently shifted its loyalty but his forces were defeated at a battle near Resaena in 243; Carrhae and Nisibis were retaken by the Romans.

With the Roman army stuck on the eastern bank of the Euphrates, Julian's successor Jovian made peace, agreeing to major concessions in exchange for safe passage out of Sasanian territory.

With both empires preoccupied by these threats, a largely peaceful period followed, interrupted only by two brief wars, the first in 421–422 after Bahram V persecuted high-ranking Persian officials who had converted to Christianity, and the second in 440, when Yazdegerd II raided Roman Armenia.

This construction project was to become a key component of the Roman defenses, and also a lasting source of controversy with the Persians, who complained that it violated the treaty of 422, by which both empires had agreed not to establish new fortifications in the frontier zone.

The proposal was initially greeted with enthusiasm by the Roman emperor and his nephew, Justinian, but Justin's quaestor, Proculus, opposed the move, believing that Khosrau's adoption would give him, and by extension Persia, a claim to the Imperial throne.

[67] Damaging raids on Syria by the Lakhmids in 529 encouraged Justinian to strengthen his own Arab allies, helping the Ghassanid leader Al-Harith ibn Jabalah turn a loose coalition into a coherent kingdom.

[77] In the wake of the Persian retreat, two Roman envoys, the newly appointed magister militum, Constantinus, and Sergius proceeded to Ctesiphon to arrange a truce with Khosrau.

The emperor seized the chance, and in 548–549 combined Roman and Lazic forces with the magister militum of Armenia Dagistheus won a series of victories against Persian armies, although they failed to take the key garrison of Petra (present-day Tsikhisdziri).

[82] In 551, general Bessas who replaced Dagistheus put Abasgia and the rest of Lazica under control, and finally subjected Petra after fierce fighting, demolishing its fortifications.

[91] Marcian's sudden dismissal and the arrival of troops under Khosrau resulted in a ravaging of Syria, the failure of the Roman siege of Nisibis and the fall of Dara.

He invaded Anatolia and sacked Sebasteia, but to take Theodosiopolis, and after a clash near Melitene the army suffered heavy losses while fleeing across the Euphrates under Roman attack and the Persian royal baggage was captured.

[95] Khosrau sought peace but abandoned this initiative when Persian confidence revived after Tamkhusro won a victory in Armenia, where Roman actions had alienated local inhabitants.

In 582, Maurice won a battle at Constantia over Adarmahan and Tamkhusro, who was killed, but the Roman general did not follow up his victory; he had to hurry to Constantinople to pursue his imperial ambitions.

[115] During these years, Heraclius strove to rebuild his army, slashing non-military expenditures, devaluing the currency and melting down Church plate, with the backing of Patriarch Sergius, to raise the necessary funds to continue the war.

The Sasanians were further weakened by economic decline, heavy taxation from Khosrau II's campaigns, religious unrest, and the increasing power of the provincial landholders.

However, the Romans repulsed the great invasion of Syria and Anatolia by Pacorus and Labienus, and were gradually able to take advantage of the weaknesses of the Parthian military system, which, according to George Rawlinson, was adapted for national defense but ill-suited for conquest.

However, the Iranian plateau never fell, as the Roman expeditions had always exhausted their offensive impetus by the time they reached lower Mesopotamia, and their extended line of communications through territory not sufficiently pacified exposed them to revolts and counterattacks.

Frye notes that in the 3rd century AD such client states played an important role in Roman–Sasanian relations, but both empires gradually replaced them by an organized defense system run by the central government and based on a line of fortifications (the limes) and the fortified frontier cities, such as Dara.

[157] Prophetically, Cassius Dio noted their "never-ending cycle of armed confrontations" and observed that "it is shown by the facts themselves that [Severus'] conquest has been a source of constant wars and great expense to us.

John F. Haldon underscores that "although the conflicts between Persia and East Rome revolved around issues of strategic control around the eastern frontier, yet there was always a religious-ideological element present".

Among them the most important are the Greeks Agathias and Malalas, the Persian Muslims al-Tabari and Ferdowsi, the Armenian Agathangelos, and the Syriac Chronicles of Edessa and Arbela, most of whom depended on late Sasanian sources, especially Khwaday-Namag.

[169] These were isolated attempts at approaching written historiography however, and by the end of the 4th century AD, even the practice of carving rock reliefs and leaving short inscriptions was abandoned by the Sasanians.