 Complete the following phrase:  Arab Muslims first attacked Sassanid territory in 633, when Khalid ibn al-Walid invaded Mesopotamia then known as the Sassanid province of Asōristān; roughly corresponding to modern-day Iraq, which was the political and economic centre of the Sassanid state. Following the transfer of Khalid to the Byzantine front in the Levant, the Muslims eventually lost their holdings to Sassanid counterattacks. The second Muslim invasion began in 636, under Sad ibn Abi Waqqas, when a key victory at the 