Abu Lu'lu'a's was either executed or committed suicide. In retaliation, Ubayd Allah ibn Umar (one of Umar's sons) killed Abu Lu'lu'a's daughter, and declared his intention to kill all non-Arabs residing in Medina. Acting upon a claim that they had been seen conspiring with Abu Lu'lu'a while he was holding the murder weapon, Ubayd Allah killed Hurmuzān, an ex-Sasanian military officer who had been working for Umar as an adviser after his capture by the Muslims, and Jufayna, a Christian man from al-Hira (Iraq) who had been taken to Medina to serve as a tutor. Just like Abu Lu'lu'a's assassination of Umar over something as trivial as a tax burden, Ubayd Allah's retaliatory killing of apparently random non-Arabs bears witness to the strong tensions that existed between Arabs and non-Arabs in the early Islamic caliphate. Uthman ibn Affan (644–656) succeeded Umar as caliph. During his reign, almost the whole of the former Sassanid empire's territory rebelled from time to time, requiring him to send several military expeditions to crush the rebellions and recapture Persia and its vassal states. The main rebellions were in the Persian provinces of Armenia, Azerbaijan, Fars, Sistan (in 649), Khorasan (651), and Makran (650). Finally, in 651, Yazdegerd III, the last Sassanid emperor, was killed near Merv by a local miller for his purse, thus putting an end to both his dynasty and to organized Persian resistance. Meanwhile, Uthman's empire expanded beyond the borders of the Sasanian Empire, to Transoxiana, Baluchistan, and the Caucasus. For many decades to come, this was the easternmost limit of Muslim rule. Persia under Muslim rule According to Bernard Lewis: Arab Muslims conquests have been variously seen in Iran: by some as a blessing, the advent of the true faith, the end of the age of ignorance and heathenism; by others as a humiliating national defeat, the conquest and subjugation of the country by foreign invaders. Both perceptions are of course valid, depending on one's angle of vision... Iran was indeed Islamized, but it was not Arabized. Persians remained Persians. And after an interval of silence, Iran reemerged as a separate, different and distinctive element within Islam, eventually adding a new element even to Islam itself. Culturally, politically, and most remarkable of all even religiously, the Iranian contribution to this new Islamic civilization is of immense importance. The work of Iranians can be seen in every field of cultural endeavor, including