The Byzantine Sassanid War of 635-650 was a conflict between the Sassanid and Byzantine emperors for hegemony over the Middle East. It started when the secret treaty between the Empire of Mecca and the Byzantine Empire came into effect after Ali's accession to the Medinan throne.
The Medinan Empire's fighting prowess was proved after the Arabian Unification Wars. The war proved the Arabic way of fighting to be supremely effective.
The Byzantine Emperor, Constans III, had already sent an alliance offer to Medina as early as 632. However, Fatimah I, afraid that the pagan tribes in Arabia would revolt in the case she went away in another war, declined Constans' deal. This lead to the antagonisation of Constans and Fatimah.
The antagonisation, however, didn't prove to be any trouble, as Fatimah died in 635 under unknown circumstances, presumably assassinated by either the pagan tribes or a Byzantine-hired assassin. Fatimah's uncle 'Ali seizes power. 'Ali, far more militaristic than his niece, declares war on the Sassanid Empire.
According to primary sources
The Persians didn't expect much more than 'cataphracts and Barbaric troops', in Sassanid Emperor Yazdegerd's own words. They were surprised when they found, im the Medinan side a quick and ruthless army, in the Byzantine one a huge army with the feared Syrian Arab warriors, Iberian and Anatolian heavy cavalry, huge infantry sources (among them the feared Brythonic, Pictish and Goidelic warriors, recruited in return for heavy gold pays to the clans, and large contingents of Slavic, Gothic and Frankish soldiers), and a huge fighting power from the Kushite side (mostly warriors recruited with their huge gold supplies). The Christian army in the war is considered either the largest military prowess on the world or an unholy coalition depending on where is the opinion asked for. The Christian army, numbering around 800,000, far outnumbered and outgunned the smaller if more cohesive Sassanid army, numbering around 350,000.
The Sassanids were also surprised, and had worse generals than the Medinan Empire (whose leader 'Ali is renowned as a great military mind). The Sassanids were definitely in a bleak position since the beginning.
'Ali made his first move breaking his army in two (giving command to the renowned general Hussayin bin Umayyad, a recent convert to Medinan Christianity) and leading his own contigent north, while Hussayin marched on Bahrayn. The Axumite emperor also surprised the Sassanid (and Medinan) army by swiftly seizing Saba. The Sassanid army, distracted in Armenia witht the huge Byzantine contingentm was forced to abandon Arabia and fall back to the Lakhmid Kingdom, where they once again prepared a defensive line.
However, the Sassanids were still to be surprised; 'Ali launched his troops and nearly obliterated the Sassanid army located there. The remains, out of all order, fled east to Mesopotamia, where they were easily defeated in several 'battles' in different cities. At last they were taken as far back as Gombroon before finally receiving a heavy Persian cavalry reinforcements, when at last the Arabs were defeated by the Persian military.
In the north, things were going similarly, with the Persian army having defeated the Byzantine one in the Baku peninsula and marching back to the border. By 640, the border had stabilized in the Tigris. The war dragged on for 10 years before finally the peace treaty.
According to contemporary historians
Although surely the Byzantine and Arabian armies were impressive, they were far from the behemoths suggested by the contemporary historians. The Byzantine army, mostly composed of western mercenaries (a group of Welsh and Pictish soldiers is more than likely), numbered between ten and three hundred thousand, whilst the Sassanid one numbered five and a hundred thousand. The Sassanid army, although still more outnumbered than the Byzantine one, had more of a chance than contemporary historians said.
Battles are remarkably well-connected to true historical remains of the battles; however, they were probably more evenly matched than just Arabs and Byzantines slaughtering fleeing Persians.
The peace between Byzantium and the Sassanid Empire caused heavy changes in the Middle Eastern map. The Sassanid empire, who had become the hegemonic power of the Middle East, became a smaller power, with Arabia and Axum gaining influence. Persia's territories in Arabia were handed to the Empire of Mecca (with the exception of Saba, granted to Ethiopia) while all of Armenia became a Byzantine vassal. The Sassanid Empire, bitter for the defeat, was soon to strike back in the War of Trebizond.