Following the orders of Yazdegerd III, Sassanid forces began to gather at the imperial capital. They came from all towns and garrisons except those manning the western frontier with the Byzantine Empire. In a few days the first army was ready. The Sassanid court expected the Muslims to proceed along the Euphrates to North-Western Iraq, as they knew that the Muslim force would not move away from the desert, which they were expected to use to retreat to, in case of defeat. Expecting the Muslim army to move west, Yazdegerd III picked Walaja as the place at which to stop Khalid ibn al-Walid and destroy his army. The first of the new Sassanid armies raised at Ctesiphon was placed under the command of Andarzaghar, governor of Khurasan province. Andarzaghar was ordered to move his army to "Walaja", where he would soon be joined by the second army. He set off from Ctesiphon, moved along the east bank of the Tigris, crossed the Tigris at Kaškar, moved south-west to the Euphrates, near Walaja, crossed the Euphrates and established his camp at Walaja.