In August 636 an Arab army destroyed the Byzantine field army in Syria at the Yarmouk, and the empire lost Syria, Palestine and eventually Egypt permanently.
Yarmouk is the most consequential battle in this archive that most readers have never heard of. In a few days in August 636 the Roman Empire lost the provinces that had been the richest part of it since Pompey, and it never got them back.
The timing is what makes it extraordinary. Heraclius had just concluded the longest and most desperate war in Byzantine history, marching into Persia and forcing a peace at Nineveh in 627 that restored every lost province. Yarmouk came nine years later.
The empire that appears in the rest of Track A — fighting on two fronts from a shrunken Anatolian base, reorganised into themes, permanently on the defensive — is the empire this battle created.
The war of 602–628 had ruined both great powers. Byzantium had won it, but had done so by stripping its provinces, melting church plate and borrowing against everything, and its frontier arrangements in the Syrian desert — the Ghassanid federates who watched the approaches — had lapsed during the Persian occupation and were not properly restored.
Arab forces took Damascus in 634 and were operating across southern Syria within two years. Heraclius, at Antioch, assembled a large army to end the problem in one campaign: Roman regulars, Armenians under Vahan, Ghassanid Arab cavalry and levies.
The Muslim commanders, hearing of it, concentrated their scattered forces on the Yarmouk — pulling back from Damascus and Emesa to do so, and reportedly returning the tribute they had taken from cities they were abandoning. Whether that gesture is history or edification, the concentration was the right decision.
The battlefield is a plain above the Yarmouk river, east of the Sea of Galilee, bounded on the west and south by the river's gorges and on the east by broken ground and a wadi.
It is open enough for large cavalry actions, which suited both armies, and enclosed enough that an army breaking on it has very little room to run — which suited only one of them.
The Byzantines drew up with their backs to that ground. That is the single fact that turns a lost battle into a destroyed army, and it is the one thing about Yarmouk that no source disputes.
The fighting lasted several days — six in the fullest accounts — and consisted of a series of Byzantine attacks that pushed the Muslim line back and were each restored before they broke it.
The mechanism was Khalid ibn al-Walid's handling of the cavalry. Instead of dividing the horse between the wings in the conventional way, he kept it concentrated as a single mobile reserve and fed it to whichever part of the line was giving way. The Byzantine army, larger but with divided command between Vahan and the imperial officers, had no equivalent.
On the last day Khalid used that reserve offensively, turned the Byzantine left, and got behind the army. The retreat ran into the ravines. The sources describe men going over the edges in the dark, and while the numbers they give are not usable, the outcome is not in question: the army did not re-form.
The simplest answers are the reliable ones. The Byzantine army was a coalition assembled for one campaign, with Armenian, Roman and Arab contingents that did not trust each other and a command split between Vahan and Theodore Trithyrios. The Muslim army had a single effective commander and had been fighting together for two years.
Beyond the battlefield, the empire's position in Syria was politically hollow. A century of imperial pressure on Miaphysite Christians had left the Syrian population with no particular attachment to Constantinople, and the Ghassanid buffer had been allowed to decay. Some Ghassanid troops are reported to have changed sides during the battle itself.
And the empire was broke. It could assemble this army once. When it was gone there was nothing behind it, which is why the loss of one battle produced the loss of an entire region rather than a second campaign.
Heraclius left Antioch for Constantinople and, in the phrase the chroniclers preserve, said farewell to Syria. Jerusalem held out until 638 and surrendered on terms; Egypt was gone by 642.
The consequences for the empire were structural rather than merely territorial. Losing Syria and Egypt cost roughly two-thirds of its revenue and its grain, and forced the whole reorganisation of the seventh century — the themes, the smaller professional armies, the abandonment of the field-army model that had just failed.
For the caliphate it opened everything. The armies that reached the walls of Constantinople in 674 and again in 717 were operating from Syria, which they held because of this battle.