The Geo-Political situation in 602 AD, just prior to the start of the grand finale to the Roman-Persian struggle, was one of tremendous polarization. The Eastern Roman Empire, comprising the Balkans, Asia Minor, the Levant, Egypt and much of North Africa was still the pre-eminent power in the Mediterranean world, even though it had wasted enormous resources in the previous century reconquering (and re-losing) much of the former Western Roman Empire. Its main competition west of Constantinople was a collection settled barbarian tribes who’s military power waxed and waned with the coming and going of powerful warlords, which meant the Romans could simply wait out much of these crises, or buy them off. However, much closer to home was the Sassanid Persian Empire, which comprised Mesopotamia, Iran, and most of Central Asia and modern Pakistan. Both of these empires were highly absolutist and rigid authoritarian systems, with powerful aristocracies and militaristic leadership. Each had very strong religious systems – Christianity for the Romans, and Zoroastrianism for the Sassanids. The empires, with their heartlands so close to this opposing force, competed with each other for a long stretch of land from the Caucausus extending into Arabia. They fought many wars against each other, and as such the one that began in 602 AD is an extension of a centuries long conflict.
So, specific background for this war. The previous war was fought for almost twenty years and largely did not get anywhere for either side. The Roman Empire ended it by supporting an exiled Persian king to the throne, who overthrew the leadership and made peace. His name was Khosrau II. He gave the Romans territory as part of the deal, and the war ended in 591. During the intervening period, the Romans were going through a financial crisis, and were incapable of holding their armies together and there were multiple mutinies during extended campaigns, the worst coming during, you guessed, 602 AD. The mutinying army declared a new emperor and overthrew the old, causing civil strife and rebellion in the outer provinces. When a Roman governor in Mesopotamia revolted, he asked for help from Khosrau II, banking on his gratitude to return a rightful emperor to Constantinople. This was a really, really, bad move. Khosrau II invaded the Roman Empire.
Things went swimmingly. Dealing with the heavily militarized frontier was a chore, but the Persians defeated Roman Empires sent to defend it and as such were able to clear out an invasion route into the empire and capture multiple important fortresses. At the same time, a man named Heraclius revolted against the current Roman Emperor (Phocas) and declared himself to be the rightful ruler of the Empire. His revolt began in Egypt and extended into Syria and Palestine, and after many victories his son (also named Heraclius) marched on Constantinople and assumed the mantle of emperor. Meanwhile the Persians were rampaging. They conquered big portions of Asia Minor and Roman Caucausian allies, and were already launching extended raids into central Asia Minor to the Bosphorus strait. Roman armies, dealing with the civil war, were largely helpless to stop the onslaught. This was by 610 AD.
Naturally, the newly crowned Roman Emperor wanted to make peace, and was willing to give significant concessions to the Persians in exchange for them leaving the Romans alone. All peace offers were rejected by Khosrau II, who saw his chance to finally conquer the only real rival the Sassanids had known and establish Sassanid hegemony over the Middle East and the Eastern Mediterranean. Thus, the war continued, and with the Persians firmly in control as they crushed Roman armies left and right and marched into the Levant, capturing Syria and Palestine, and in the process laid waste to Jerusalem in 614 AD, and capturing the True Cross as a victory trophy. The Romans were largely incapable of any organized resistance, as the territory under their control diminished by the day. In 616 AD, the Sassanids then invaded Egypt with their best general, a man named Farrohkhan with the title Shahrbaraz (meaning the Boar of the Empire). Egypt was of enormous importance to the Romans, as Egypt was the breadbasket of the Mediterranean world and fed much of the Roman Empire.
It fell by 618 AD. So how much of the Roman Empire was left? About half of Anatolia, and the Balkans. And Anatolia was already being invaded. Oh, and so were the Balkans, by Slavic tribes.
But if you ever learn one thing about Rome, and the Roman Empire, learn this: They had grit. A lot of it.
Roman currency in this period said “May God help the Romans.” God isn’t real, but the Romans had Heraclius. He instituted massive austerity measures to pay for the war, slashing government expenditures and looting everything his tax collectors could find that could be used in coinage. Churches, old monuments, etc were stripped of valuables to be melted down for coinage. He even began to frame it as a war for the defense of Christianity. He put together one last army to throw out the Persians in 622 AD, and he led it personally. After Easter Mass his army marched out and proceeded to destroy Shahrbaraz’s army at Issus and ending the Persian attack into Anatolia. Heraclius then paid off the Slavic tribes and stalled an invasion from them in 623 AD. The war had gone on for 21 years, and left most of the Eastern Roman Empire in shambles or under Persian control. Heraclius looked at his domain, his outnumbered armies and shattered economy, and went on the offensive into Persia in 624 AD.
Heraclius’ army recaptured the Cauacasian territories, and proceeded to systematically destroy every Persian army thrown against them. Shahrbaraz (in charge in this theatre after his defeat at Issus) was defeated over and over, retreating deeper and deeper into Mesopotamia. The Romans left a wide swath of destruction and now were in a position to potentially get a negotiated settlement. Khosrau II was most displeased. A smaller Roman army had just defeated multiple much larger Persian forces and had touched the heartland of the empire. So he raised a huge army and negotiated with the slavs that Heraclius had bribed. Constantinople was then besieged in 627 AD. The Romans, however, never doubted. At this point they were fanatically religious and tired of these fucking Persians in their empire. They held, the slavs were defeated and the Persian armies destroyed. Khosrau was really pissed and ordered Shahrbaraz put to death, which Shahrbaraz heard about and then absconded with his army and defected. At the same time the Romans decided they could play at the barbarian game and had their friends the Turks invade the Persians and mess around there awhile.
In winter 627 AD the Romans invaded Persia and ended the war as the Persians had no armies left to fight. Khosrau was overthrown in 628 AD, all territories were returned, and a triumphal procession was held in Jerusalem as the True Cross came back into Roman possession. The Romans had triumphed against completely absurd odds and destroyed their dangerous opponent, and their only real competitor.
Of course, the Muslim Arabs invaded in 634 AD and conquered almost the entirety of the Eastern Roman Empire and totally destroyed the Sassanids by 651 AD.