Roman Republic no relation Fall of the Sassanid Empire
The Roman–Persian Wars, also known as the Roman–Iranian Wars, were a series of conflicts between states of the Greco-Roman world and two successive Iranian empires: the Parthian and the Sasanian. Battles between the Parthian Empire and the Roman Republic began in 54 BC;  wars began under the late Republic, and continued through the Roman (later Eastern Roman (Byzantine)) and Sasanian empires. A plethora of vassal kingdoms and allied nomadic nations in the form of buffer states and proxies also played a role. The wars were ended by the early Muslim conquests, which led to the fall of the Sasanian Empire and huge territorial losses for the Byzantine Empire, shortly after the end of the last war between them.
Although warfare between the Romans and Persians continued over seven centuries, the frontier, aside from shifts in the north, remained largely stable. A game of tug of war ensued: towns, fortifications, and provinces were continually sacked, captured, destroyed, and traded. Neither side had the logistical strength or manpower to maintain such lengthy campaigns far from their borders, and thus neither could advance too far without risking stretching its frontiers too thin. Both sides did make conquests beyond the border, but in time the balance was almost always restored. Although initially different in military tactics, the armies of both sides gradually adopted from each other and by the second half of the 6th century, they were similar and evenly matched.The expense of resources during the Roman–Persian Wars ultimately proved catastrophic for both empires. The prolonged and escalating warfare of the 6th and 7th centuries left them exhausted and vulnerable in the face of the sudden emergence and expansion of the Rashidun Caliphate, whose forces invaded both empires only a few years after the end of the last Roman–Persian war. Benefiting from their weakened condition, the Rashidun armies swiftly conquered the entire Sasanian Empire, and deprived the Eastern Roman Empire of its territories in the Levant, the Caucasus, Egypt, and the rest of North Africa. Over the following centuries, more of the Eastern Roman Empire came under Muslim rule.Aftermath
The devastating impact of this last war, added to the cumulative effects of a century of almost continuous conflict, left both empires crippled. When Kavadh II died only months after coming to the throne, Persia was plunged into several years of dynastic turmoil and civil 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. The Byzantine Empire was also severely affected, with its financial reserves exhausted by the war and the Balkans now largely in the hands of the Slavs. Additionally, Anatolia was devastated by repeated Persian invasions; the Empire's hold on its recently regained territories in the Caucasus, Syria, Mesopotamia, Palestine and Egypt was loosened by many years of Persian occupation.Neither empire was given any chance to recover, as within a few years they were struck by the onslaught of the Arabs (newly united by Islam), which, according to Howard-Johnston, "can only be likened to a human tsunami". According to George Liska, the "unnecessarily prolonged Byzantine–Persian conflict opened the way for Islam". The Sasanian Empire rapidly succumbed to these attacks and was completely conquered. During the Byzantine–Arab wars, the exhausted Roman Empire's recently regained eastern and southern provinces of Syria, Armenia, Egypt and North Africa were also lost, reducing the Empire to a territorial rump consisting of Anatolia and a scatter of islands and footholds in the Balkans and Italy. These remaining lands were thoroughly impoverished by frequent attacks, marking the transition from classical urban civilization to a more rural, medieval form of society. However, unlike Persia, the Roman Empire ultimately survived the Arab assault, holding onto its residual territories and decisively repulsing two Arab sieges of its capital in 674–678 and 717–718. The Roman Empire also lost its territories in Crete and southern Italy to the Arabs in later conflicts, though these too were ultimately recovered.
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