The Battle of al-Q disiyyah was the decisive engagement between the Arab Muslim army and the Sassanid Persian army during the first period of Muslim expansion which resulted in the Islamic conquest of Persia. It is regarded as the decisive battle in the Islamic conquest of Persia, and a key to the conquest of Iraq. The battle also saw the alleged alliance of Emperor Yazdegerd III with Emperor Heraclius, who then married off his grand daughter Manyanh to Yazdegerd as a symbol of alliance.