In 635 Yazdgerd III sought an alliance with Emperor Heraclius of the Eastern Roman Empire, marrying the latter's daughter (or, by some traditions, his granddaughter) in order to seal the arrangement. While Heraclius prepared for a major offence in the Levant, Yazdegerd ordered the concentration of massive armies to push the Muslims out of Mesopotamia for good through a series of well-coordinated attacks on two fronts.