Mu´tah traditionally dated 629, but this is not certain. The first engagements may have started as conflicts with the Arab client states of the Byzantine and Sassanid empires: the Ghassanids and the Lakhmids of Al-Hirah. In any case, Muslim Arabs after 634 certainly pursued a full-blown offensive against both empires, resulting in the conquest of the Levant, Egypt and Persia for Islam. The most successful Arab generals were Khalid ibn al-Walid and 'Amr ibn al-'As. Arab conquest of Roman Syria: 634–638 In the Levant, the invading Rashidun army were engaged by a Byzantine army composed of imperial troops as well as local levies. According to Islamic historians, Monophysites and Jews throughout Syria welcomed the Arabs as liberators, as they were discontented with the rule of the Byzantines. The Roman Emperor Heraclius had fallen ill and was unable to personally lead his armies to resist the Arab conquests of Syria and Roman Paelestina in 634. In a battle fought near Ajnadayn in the summer of 634, the Rashidun Caliphate army achieved a decisive victory. After their victory at the Fahl, Muslim forces conquered Damascus in 634 under the command of Khalid ibn al-Walid. The Byzantine response involved the collection and dispatch of the maximum number of available troops under major commanders, including Theodore Trithyrius and the Armenian general Vahan, to eject the Muslims from their newly won territories. At the Battle of Yarmouk in 636, however, the Muslims, having studied the ground in detail, lured the Byzantines into pitched battle, which the Byzantines usually avoided, and into a series of costly assaults, before turning the deep valleys and cliffs into a catastrophic death-trap. Heraclius' farewell exclamation (according to the 9th-century historian Al-Baladhuri) while departing Antioch for Constantinople, is expressive of his disappointment: "Peace unto thee, O Syria, and what an excellent country this is for the enemy!" The impact of Syria's loss on the Byzantines is illustrated by Joannes Zonaras' words: "[...] since then [after the fall of Syria] the race of the Ishmaelites did not cease from invading and plundering the entire territory of the Romans". In April 637 the Arabs, after a long siege, captured Jerusalem, which was surrendered by Patriarch Sophronius. In the summer of 637, the Muslims conquered Gaza, and, during the same period, the Byzantine authorities in Egypt and Mesopotamia purchased an expensive truce, which lasted three years for Egypt and one year for Mesopotamia. Antioch