Roman–Persian wars

The Roman–Persian wars, also called the Roman–Iranian wars, took place between the Greco-Roman world and the Iranian world, beginning with the Roman Republic and the Parthian Empire in 54 BC and ending with the Roman Empire (including the Byzantine Empire) and the Sasanian Empire in 628 AD. 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. The Rashidun offensives resulted in the collapse of the Sasanian Empire and largely confined the Byzantine Empire to Anatolia for the ensuing Arab–Byzantine wars.

Aside from shifts in the north, the Roman–Persian border remained largely stable for the duration of the conflict, albeit subject to an effective tug of war: towns, fortifications, and provinces were continually sacked, captured, destroyed, and traded, but 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 their 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 Romans and the Persians gradually adopted from each other, and by the second half of the 6th century, they were similar and evenly matched.

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. Benefiting from the two empires' weakened conditions, the Rashidun Caliphate swiftly annexed Persia on the Sasanian front; and the Levant, the Caucasus, and Egypt and the rest of North Africa on the Byzantine front.