It is a common mistake to think of history in terms of battles and wars alone, as if the other events occurring in the world don’t matter. In general, however, the history of ancient times is really a history of constant war. What can define an era, then, is who the wars are against. The history of Rome, for example, can be viewed as fundamentally shifting following the defeat of Carthage, when the Roman Republic which had fought against this rival for so long found that it stood alone. In this case, the enemy they turned to fight were themselves, leading to the first (of many) Roman civil wars. A similar shift was about to occur to Byzantium in 636, where one defeated enemy would soon be replaced by another – although in this case, the new enemy would prove to be both external, and far deadlier than the first.
Through the early part of their history, the Byzantines were defined by their war with Persia. The first conflict between the two took place in the dying days of the Roman Republic, and the death of the Triumvir Crassus in that battle led directly to the civil war between Pompey and Caesar that would lead to the end of Roman democracy. The last battle between the two came at the conclusion of what scholars refer to as the Last Great War Of Antiquity (at least partially because it _was_ the final Roman/Persian conflict), where they finally defeated the Persian army and stood poised to take over the entire rival empire. Or so they thought.
Far to the south of this conflict stood the Arabian Peninsula, home to innumerable desert tribes. They had not made any major impact on the world up until this point. Although the Romans had once had a province there, it was too far from the centre to hold onto long, and the area was generally beneath the notice of the great empires. Meanwhile, their own disunity and constant intertribal warfare without any outside threats to unite them left them isolated. Until, that is, the arrival of Islam. The new religion gave coherence and unity to the Arab peoples, along with a drive that would see them explode onto the world stage in dramatic fashion.
While the Byzantines had fought the Arabs in some skirmishes and minor battles, their first real engagement was at the Battle of Firaz, a battle in Mesopotamia where a small Byzantine force actually allied with their old enemies, the Persians. The Arab forces under the command of legendary general Khalid ibn al-Walid easily defeated the larger mixed force, through a combination of shrewd maneouvres and exploiting the disunity between the froces on the opposite side. After the Arabs went on to conquer the rest of Persia at the Battle of al-Qādisiyyah, the Emperor Heraclius began to panic. He dispatched the might of the legions to check the seeming inexorable Arab advance. The Byzantines moved south as the Arabs moved west, and the two armies met at the site of the dried-up Yarmouk river.
The Byzantines may still have been Roman Legionnaires at heart, but they bore little resemblances to our traditional view of the Roman warriors. They wore elaborate armour and wielded curved eastern blades, and had even adopted archery, a role previously relegated to units of foreign mercenaries. They were disciplined warriors, and probably thought themselves more than a match for this rabble from the desert. Unfortunately for them, they were facing possibly the greatest military genius of the age. Khalid had studied the Byzantines and used them as a pattern for his own reorganisation of the rag-tag Arab forces into an army that would conquer half the known world. He was about to show the Byzantines just how deadly their own tactics could be.
Khalid knew that the Byzantine army had a larger corp of more experienced officers than his own. He did have a counter to that greater experience however – the Mubarizun. Specially trained duellists, these warriors were used by Khalid to take advantage of the Byzantine and Sassanid tradition of duelling before the battle proper, and the vanity of the Byzantine leaders. One Arab warrior advanced out, dropped his shield and removed his armour, crying out “I am the death of the Pale Faces, I am the killer of Romans, I am the scourge sent upon you, I am Zarrar Ibn al-Azwar!” Several Byzantine officers challenged him, but all perished. He and the other Mubarizun not only left the Byzantines without the leaders they needed for the upcoming battle, but also presumably left the rank and file utterly shitting themselves at the prospect of facing forty thousand of these maniacs.
Despite this, the battle seemed evenly matched, with the Romans gaining an early advantage due to their superior numbers. They even forced an Arab retreat, back to their camp, where the leader of the womanfolk, a formidable lady named Hind who had once led an army against Muhammed, organised the defence that prevented Byzantine victory. The main thing that won the battle for the Arabs, however, was Khalid’s use of his cavalry. By organising them into a mobile guard division, rather than allocating them specific positions, he was able to reinforce key areas, or turn advantages into victory in others. His cavalry also had a major advantage over the Romans as they rode camels, and the Roman horses would go into a panic at the smell of them. On the sixth day, his cavalry managed to rout the Byzantine cavalry entirely, which allowed them to attack the Roman rear, forcing a general retreat. Khalid funnelled them towards a bridge that his forces had secretly captured earlier. The Byzantines were slaughtered, and the few survivors of the cream of the legions fled the field in disarray.
Byzantium would survive another eight centuries, and remain a major world power for four of those, but the defeat at Yarmouk turned what could have been the dawning of a new age of Roman dominance over the known world into a mere footnote to the rise of a new great power. Persia, rather than becoming part of the Roman Empire, became part of the new Umayyad Caliphate. The Arabs would conquer the whole of the Middle East, the provinces of North Africa that had once been Roman, and even make a good stab at conquering the whole of Europe. The Byzantine Empire had not defeated a rival, but merely replaced them with a new, more inexorable one.