a threat, and he refused to send an army to encounter the invaders. When the main Arab army reached the Persian borders, Yazdegerd III procrastinated in dispatching an army against the Arabs. Even Rostam-e Farokhzad, who was both Eran Spahbod and Viceroy, did not see the Arabs as a threat. Without opposition, the Arabs had time to consolidate and fortify their positions. When hostilities between the Sasanians and the Arabs finally began, the Persian army faced fundamental problems. While their heavy cavalry had proved effective against the Roman forces, it was too slow and regimented to act with full force against the agile and unpredictable lightly armed Arab cavalry and foot archers. The Persian army had a few initial successes. War elephants temporarily stopped the Arab army, but when Arab veterans returned from the Syrian fronts where they had been fighting against Byzantine armies, they taught the Arab army how to deal with these beasts. These factors contributed to the decisive Sassanid defeat at the Battle of al-Qādisiyyah. The Persians, who had only one generation before conquered Egypt and Asia Minor, lost decisive battles when nimble, lightly armed Arabs accustomed to skirmishes and desert warfare attacked them. The Arab squadrons defeated the Persian army in several more battles culminating in the Battle of Nahāvand, the last major battle of the Sassanids. The Sassanid dynasty came to an end with the death of Yazdegerd III in 651. Rise of the Caliphate Muhammad died in June 632, and Abu Bakr took the title of Caliph and political successor at Medina. Soon after Abu Bakr's succession, several Arab tribes revolted, in the Ridda Wars (Arabic for the Wars of Apostasy). The Ridda Wars preoccupied the Caliphate until March 633, and ended with the entirety of the Arab Peninsula under the authority of the Caliph at Medina. Abu Bakr set in motion a historical trajectory (continued later by Umar and Uthman) that in a few decades led to one of the largest empires in history, beginning with a confrontation with the Sassanid Empire under the general Khalid ibn al-Walid. First invasion of Mesopotamia (633) After the Ridda wars, a tribal chief of northeastern Arabia, Al-Muthanna ibn Haritha, raided the Sasanian towns in Mesopotamia (modern-day Iraq). Abu Bakr was strong enough to attack the Persian Empire in the north-east and the Byzantine Empire in the north-west. There were three purposes for this conquest. First, along