When Khalid ﵁ received the orders to push through to the northeast, he was still reorganizing in al-Yamamah. Abu Bakr ﵁ had written to him that al-Muthanna ibn Haritha ash-Shaybani ﵁ had for months been conducting skirmishes along the Sasanian border, and that a coordinated campaign was needed. Khalid ﵁ was to join al-Muthanna ﵁ with his main force, thrust into southern Iraq, and threaten the Lakhmid capital of Hira.
The geography was new to the Arabs of the peninsula. The Euphrates delta, the salt marshes around Basra, the irrigation canals that crossed like a woven fabric, the palm groves that offered shade to the horizon. Khalid ﵁ chose a swift approach from the desert. He assembled eighteen thousand men on the edge of the Nafud, and he sent a letter ahead to Hormuz, the Sasanian marzban of the southern front at Kazima.
The letter was short. “Accept Islam, and you shall have safety. Refuse, and pay the jizyah. Refuse that too, and hold yourself ready for men who love death more than you love life.”
Hormuz laughed. He was a military governor appointed by the shah, a man with a golden headdress, his own palatine guard, and a Sasanian conception of war that saw the Arab bedouin as disorderly raiders. He gathered his troops at Kazima, on the Persian Gulf, and he had chains locked around the ankles and wrists of his elite infantry. The chains, in groups of ten, meant that the first rank could not flee. Whoever stood firm held the whole row standing. Whoever fled dragged nine others with him.
Khalid ﵁ arrived in the early morning. He deployed his cavalry on both flanks and his infantry in the center. Hormuz, in keeping with Sasanian custom, called for a mubaraza, a duel between the two commanders before the main battle began. Khalid ﵁ rode forward. Hormuz rode forward. The Sasanian commanders made a tradition of poisoned daggers used at the last moment in a mubaraza. One of his officers stood ready to attack Khalid ﵁ from behind during the duel.
The duel lasted less than a minute. Khalid ﵁ seized Hormuz in a wrestling grip, flung him aside, and killed him with a single blow to the throat. At that same moment al-Qaʿqaʿ ibn ʿAmr ﵁, Khalid ﵁'s bodyguard, sprang forward and killed the officer who had meant to strike treacherously from behind.
With Hormuz dead and his chained infantry powerless to give way, the Sasanian line broke. The flight became a rout. The chained units could not flee and were cut down where they stood. Khalid ﵁ had the remaining chains sent to Medina as a trophy. Abu Bakr ﵁ awarded the golden headdress of Hormuz, according to tradition, to Khalid ﵁ as his personal share, and thereby gave the campaign his blessing.
Kazima fell within a day. Khalid ﵁ rested two days, tended the wounded, and set course north, up the Euphrates, where new Sasanian forces were gathering to stop him.