The Enemy
Khālid was of Makhzūm, the clan that had led the persecution, and was the best cavalry commander in Arabia. At Uḥud he commanded the Makkan right wing, saw the Muslim archers abandon the hill after the enemy line broke, and took his horsemen round behind them. It was the single manoeuvre that turned a Muslim victory into their worst defeat, and nearly killed the Prophet ﷺ.1
He was on the Makkan side again at the Trench. He was present at Ḥudaybiyyah as a commander of Qurayshī cavalry sent to intercept the pilgrims.1
The Conversion
In the year after Ḥudaybiyyah he came to Madīnah with ʿAmr ibn al-ʿĀṣ and ʿUthmān ibn Ṭalḥah and accepted Islam. His brother al-Walīd had written to him that the Prophet ﷺ had asked after him and said, “A man like Khālid — how can Islam be hidden from him?” Khālid said that this letter was what moved him.2
The Prophet ﷺ received him and, when Khālid asked forgiveness for what he had done, told him that Islam wipes out what came before it. Within months he was leading Muslim armies.2
At Muʾtah, when all three appointed commanders had been killed in succession, the men gave the banner to Khālid. He extricated three thousand men from a vastly larger force — a retreat that the sources treat as an achievement rather than a defeat. The Prophet ﷺ, announcing it in Madīnah, said the banner was taken up by “a sword among the swords of God,” and it is from that day that the title Sayf Allāh attached to him permanently.3 Khālid himself said that nine swords broke in his hand that day.3
The Commander
Under Abū Bakr he broke the apostasy in central Arabia and killed Musaylimah's forces at al-Yamāmah; under ʿUmar he took the Persian frontier and then crossed the desert to Syria in a march the historians still discuss, defeated the Byzantines at Ajnādayn, took Damascus, and destroyed the imperial army at the Yarmūk in 636 — the battle that ended Byzantine rule in Syria.4
The sīrah does not present him as beyond criticism. Sent to Banū Jadhīmah after the conquest of Makkah, he killed men who had surrendered on a misunderstanding of their words; when the Prophet ﷺ was told, he raised his hands and said twice, “O God, I am innocent before You of what Khālid has done,” and sent ʿAlī with money to pay compensation for every life and every loss, down to the value of a dog's water-trough.5
ʿUmar dismissed him from command at the height of his fame — the sources give as the reason ʿUmar's fear that people were attributing victory to Khālid rather than to God. He obeyed, served in the ranks under the man who replaced him, and died in his bed in Ḥimṣ. He is reported to have said: “I have fought in so many battles, and there is not a hand's span on my body but has a wound from a sword or an arrow — and here I die in my bed as a camel dies. May the eyes of the cowards never rest.”6