Khalid then left Mesopotamia to lead another campaign in Syria against the Byzantine Empire, after which Mithna ibn Haris took command in Mesopotamia. The Persians once again concentrated armies to regain Mesopotamia, while Mithna ibn Haris withdrew from central Iraq to the region near the Arabian desert to delay war until reinforcement came from Medina. Umar sent reinforcements under the command of Abu Ubaidah Saqfi. With some initial success this army was finally defeated by the Sasanian army at the Battle of the Bridge in which Abu Ubaid was killed. The response was delayed until after a decisive Muslim victory against the Romans in the Levant at the Battle of Yarmouk in 636. Umar was then able to transfer forces to the east and resume the offensive against the Sasanians. Umar dispatched 36,000 men along with 7500 troops from the Syrian front, under the command of Sa`d ibn Abī Waqqās against the Persian army. The Battle of al-Qādisiyyah followed, with the Persians prevailing at first, but, on the third day of fighting, the Muslims gained the upper hand. The legendary Persian general Rostam Farrokhzād was killed during the battle. According to some sources, the Persian losses were 20,000, and the Arabs lost 10,500 men.