The battle of Kadisiya (Qadisiyya, Qadisiyyah) was the decisive engagement between the Arab invaders and the Persian army during the first period of Islamic expansion wich resulted in the Islamic conquest of Iran.

The growing power of the Arabian Moslems made certain that a showdown with the empire of Sassanid Persia to the east would occur, but the Persians moved first. In the spring of 637, Rustam , regent for Yazdegerd III, led an army of about 100,000 men across the Euphrates River to Kadisiya (near the modern Hilla in Iraq). Expecting the Persian attack, Caliph Umar ibn al-Khattab sent forth 30,000 Arabian cavalrymen under Sa'ad ibn-Abi-Waqqas .

The battle began with the usual series of cavalry rushes by the Arabs. But the huge Persian force held its ground and then counterattacked with elephants, which terrified the Arabian horses. Sa'ad was barely able to prevent a rout at the end of the first day's combat. Fighting resumed the second day, although the slashing Moslem attacks inflicted heavier casualties than were received. On the third day Sa'ad was reinforced by some veterans of the Syrian campaign who knew how to fight elephants with arrows and javalins. The beasts were wounded and then stampeded back through the Persian lines, opening holes for the Arabian cavalry to charge through. The Moslems pressed home their attacks throughout the day and during the night (called the "Night of the Clangor").

At daybreak, a sandstorm began blowing in the faces of the stubborn Persians. Rustam sought personal safety by swimming across a canal running to the Euphrates. He was caught and beheaded. The Persian army then disintegrated, taking terrible losses from the Arabians, who gave no quarter. Moslem losses in the battle totaled 7,500 killed. In the booty captured by the Arabs was the jewel-encrusted sacred banner of Persia.

Sa'ad crossed the Euphrates in pursuit. Yazdegerd offered to yield all territory west of the Tigris River. When the Arabs declined his offer, the Persian emperor abandoned his capital at Ctesiphon, which was promptly occupied and sacked. Yazdegerd made two further counterattacks to halt the Moslem invaders, both in vain.