After Khalid left Iraq for Syria, Suwad, the fertile area between the Euphrates and the Tigris, remained unstable. Sometimes it was occupied by the Persians and sometimes by the Muslims. This "tit-for-tat" struggle continued until emperor Yazdegerd III consolidated his power and sought an alliance with Heraclius in 635 in an effort to prepare for a massive counterattack. Heraclius married his granddaughter, Manyanh, to Yazdegerd III, in accordance with Roman tradition to seal an alliance. Heraclius then prepared for a major offensive in the Levant. Meanwhile, Yazdegerd ordered a concentration of massive armies to reclaim Iraq for good. This was supposed to be a well-coordinated attack by both emperors to annihilate the power of their common enemy, Caliph Umar.
