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 Sa'd ibn Abi Waqqas, when a key victory at the Battle of al-Qadisiyyah led to the permanent end of Sassanid control west of Iran. For the next six years, the Zagros Mountains, a natural barrier, marked the border between the Rashidun Caliphate and the Sassanid Empire. In 642, the Muslim Caliph Umar ordered a full-scale invasion of Persia by the Rashidun army, which led to the complete conquest of the Sassanid Empire by 651. Directing from Medina, a few thousand kilometres away, Umar's quick conquest of Iran in a series of well-coordinated, multi-pronged attacks became his greatest triumph, contributing to his reputation as a great military and political strategist. In 644, prior to the complete annexation of Persia by the Rashidun Arabs, Umar was assassinated by Piruz Nahavandi, a Sassanid Persian soldier who was captured in battle and brought to Arabia as a slave.