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In the mid-11th century, England faced a severe political conflict over the royal succession. Although Edward the Confessor ascended the throne in 1042, he had no children, intensifying the competition among nobles for the crown. Edward maintained a close relationship with William, Duke of Normandy, who claimed he had been promised the English throne. However, Harold Godwinson, a prominent English nobleman, was also a strong contender. When Edward died in January 1066, Harold, with the support of English nobles, was crowned king. William refused to accept this, asserting that his claim to the throne had been disregarded. Meanwhile, Harald Hardrada of Norway also laid claim to the English throne and prepared to invade. Thus, in 1066, England found itself in a situation where multiple powers clashed over the issue of royal succession.

The rivalry between Rome and Persia came to an end in the mid-7th century with the fall of the Sasanian Empire. The Persian Empire vanished from history with the death of the last Sasanian monarch, Yazdegerd III, in 651. The Eastern Roman Empire was also reorganized into a state centered in Anatolia, having lost significant portions of Syria, Egypt, and North Africa. The two empires clashed repeatedly for centuries over Mesopotamia, Armenia, Syria, and the Caucasus. Neither side could completely conquer the other, and war and truce alternated continuously. The war of 602-628, in particular, severely depleted the manpower and finances that both nations could mobilize, and shortly thereafter, the Arab conquests began, leading to the collapse of the existing international order. The Roman-Persian Wars were more than just border conflicts; they were a historical process of long-term competition between Roman and Iranian civilizations. During this process, military systems, diplomacy, fortress construction, and trade routes developed, leaving a profound impact on the politics and culture of the Middle East thereafter.

With Heraclius' counterattack, Rome regained most of the eastern territories it had lost, but the prolonged war significantly weakened both empires. After Khosrow II was deposed in 628, the Sasanian dynasty fell into a power struggle and political turmoil. The newly empowered Kavad II made a peace treaty with Rome and returned the occupied territories. Heraclius restored Roman rule in Syria, Egypt, and Mesopotamia. However, shortly after achieving victory, a new threat emerged. The Islamic forces rising from Arabia rapidly expanded and began attacking the borders of both empires. After 634, Arab armies attacked Syria and Palestine, and the Roman army suffered a major defeat at the Battle of Yarmouk. The Sasanian dynasty also lost territory after defeats at the Battle of Qadisiyyah and the Battle of Nihavand. The long Roman-Persian wars ultimately exhausted both nations, leaving them in a state where they could not effectively respond to the attacks of the new power.

In the late 6th century, relations between Rome and the Sasanian Empire deteriorated again. When the war began in 572, both nations fought fiercely in Armenia and Mesopotamia. The conflict lasted for decades, with both sides committing vast amounts of troops and resources. However, the situation changed dramatically with the accession of Emperor Phocas in 602. Khosrow II of the Sasanian Empire, using his relationship with the previous emperor Maurice as a pretext, attacked Rome. The Sasanian army rapidly advanced into Syria, Egypt, and parts of Anatolia, at one point even threatening the safety of Constantinople. But Heraclius, who became emperor in 610, reorganized the military and finances and launched a major counteroffensive. He employed mobile warfare, attacking the enemy's rear and pressuring the Sasanian forces, ultimately achieving victory at the Battle of Nineveh in 627. Following this, internal turmoil grew within the Sasanian Empire, leading to the deposition of Khosrow II and the restoration of peace between the two nations.

The Roman-Persian Wars during Emperor Justinian's reign were central to the Eastern policy of the 6th century. After 527, both empires clashed in the Caucasus and Mesopotamia, leading to a temporary peace treaty in 532. However, the war reignited in 540 when Khosrow I of the Sasanian Empire invaded Syria. The Sasanian forces captured and plundered Antioch, prompting Rome to invest heavily in its defense. The conflict subsequently continued for an extended period, primarily focused on the Caucasus and Mesopotamia. Justinian had to manage multiple fronts simultaneously, utilizing Belisarius and other generals. Finally, a peace treaty was signed in 562, bringing a truce between Rome and the Sasanian Dynasty. Rome secured peace on its borders by paying a significant annual sum. However, this agreement was not a permanent solution. Both empires remained wary of each other, and the balance of power in Armenia and the Caucasus continuously strained their relationship.

In the 5th century, as the Western Roman Empire weakened, clashes between Rome and the Sasanian Empire temporarily decreased. The Eastern Roman Empire, while maintaining control over the Balkans, Egypt, and Anatolia, avoided war with Persia and focused on border defense. The two powers occasionally clashed over influence in Armenia and the Caucasus region, but also had to contend with other threats emerging from Central Asia and the north. Even after the collapse of the Western Roman Empire at the end of the 5th century, the Eastern Roman Empire continued the Roman state system. However, the Sasanian Empire remained a formidable military power, so the rivalry between the two empires did not end. During the reigns of Justin I and Justinian I in the early 6th century, relations with Persia became tense again. Control over the regions of Lazica and Armenia became a particularly important issue. From this point on, the wars became longer and more complex, significantly impacting the Eastern Roman Empire's strength and diplomatic strategy.

In the 4th century, the rivalry between Rome, which had adopted Christianity, and the Sasanian Empire, centered around Zoroastrianism, began to intertwine with religious issues. As Christianity's status significantly rose within the Roman Empire after the reign of Constantine the Great, the Sasanian Empire grew suspicious of its own Christians as potential pro-Roman elements. Concurrently, conflicts over the control of Armenia and Mesopotamia persisted. Shapur II engaged in prolonged warfare with Rome, and Emperor Julian launched a large-scale expedition in 363. The Roman army advanced near Ctesiphon but could not push further due to supply issues and Sasanian resistance. After Julian was killed in battle, his successor Jovian retreated the army and signed a peace treaty with the Sasanian Empire. This treaty led Rome to cede some eastern territories, and the borders were readjusted. The wars of the 4th century confirmed that neither nation could easily conquer the other.

As the Sasanian pressure intensified, Rome faced a severe crisis in the mid-3rd century. Shapur I exploited Rome's chaos to attack Mesopotamia, and in 260, Emperor Valerian was defeated at the Battle of Edessa and captured. This was a deeply shocking event in Roman history. However, Rome soon counterattacked. Odaenathus, the leader of Palmyra, attacked the Sasanian forces, contributing to the recovery of Roman territories in the East. Emperor Aurelian later reunified the empire. By the end of the 3rd century, Diocletian strengthened the eastern defenses and reorganized the border fortresses and administrative system. In 298, Galerius defeated the Sasanian army, securing favorable terms for Rome. The Treaty of Nisibis strengthened Roman influence in Armenia and ceded parts of Mesopotamia to Rome. However, this peace did not last long.

With the fall of the Parthians and the rise of the Sasanian dynasty in the early 3rd century, the Roman eastern front entered a new phase. In 224 AD, Ardashir I overthrew the last Parthian dynasty and established a strong, centralized state. The Sasanian dynasty, claiming to inherit the former territories and authority of the Achaemenid Empire, expanded its influence in Mesopotamia and Armenia. Clashes with Rome soon followed. In the 230s, Sasanian forces attacked Roman territories in Mesopotamia, prompting Roman Emperor Alexander Severus to launch a campaign. Thereafter, both nations continued to fight over cities and fortresses between the Euphrates and Tigris rivers. From this period, the conflict evolved beyond simple border disputes into a long-term rivalry between the two empires, each seeking to assert its sphere of influence. Armenia, in particular, repeatedly became a battlefield between these two great powers.

The conflict between Rome and Parthia continued even after the Roman Empire began. In the 1st century AD, who would control the Armenian throne became a key issue. During Emperor Nero's reign, Roman general Corbulo was dispatched to the East to wage war over Armenia, and the crisis was eventually resolved through a compromise between Rome and Parthia. Later, Emperor Trajan pursued a more aggressive policy. In 113 AD, he attacked Armenia and Mesopotamia, capturing the capital Ctesiphon and advancing to the Persian Gulf. However, the conquered territories were vast, and with continuous rebellions, Roman rule did not last long. Upon his accession, Hadrian abandoned Trajan's conquests and retreated to the Euphrates frontier. This process demonstrated that it was more important for Rome to stably maintain its existing borders than to expand its territory in the East.