The Battle of Yarmuk
The Muslims, around thirty-six thousand, met the massed Byzantines — over two hundred thousand — on the banks of the Yarmuk. Allah decreed for them a decisive victory that expelled the Byzantines from Syria for good.
Heraclius mustered his greatest armies to recover Syria, so the Muslims drew back to the plain of Yarmuk. Khalid ibn al-Walid united the armies and divided them into companies, and spoke his words: "This is a day of the days of Allah; there should be neither boasting nor transgression in it."
The Muslims withstood the crushing Byzantine assaults for days; the women stood behind the ranks turning back any who fled, until Khalid launched his general attack, closing upon the Byzantines and driving them to the ravine of al-Waqusa, where thousands of them plunged into its gorges.
Tens of thousands of Byzantines were killed and about three thousand Muslims were martyred, among them ʿIkrima ibn Abi Jahl. Heraclius spoke his famous farewell: "Peace be upon you, O Syria — the farewell of one who does not return." Yarmuk was the day the Byzantines left Syria forever.
Note — differing reports on the date: The well-known view of Ibn Ishaq and al-Tabari places it in Rajab 15 AH; others, such as Sayf ibn ʿUmar, place it in 13 AH, immediately after Ajnadayn.