Arab Muslim forces. Many localities fought against the invaders; although Arabs had established hegemony over most of the country, many cities rose in rebellion by killing their Arab governors or attacking their garrisons. Eventually, Arab military reinforcements quashed the Iranian insurgencies and imposed complete Islamic control. The Islamization of Iran was gradual and incentivized in various ways over a period of centuries with some Iranians never converting and widespread cases of Zoroastrian scriptures being burnt and priests being executed, particularly in areas that experienced violent resistance. The Persians began to reassert themselves by maintaining the Persian language and Iranian culture. Islam became the dominant religion in Iran by the late Middle Ages. Historiography and recent scholarship When Western academics first investigated the Muslim conquest of Persia, they relied solely on the accounts of the Armenian Christian bishop Sebeos, and accounts in Arabic written some time after the events they describe. The most significant work was probably that of Arthur Christensen, and his L’Iran sous les Sassanides, published in Copenhagen and Paris in 1944. Recent scholarship has begun to question the traditional narrative: Parvaneh Pourshariati, in her Decline and Fall of the Sasanian Empire: The Sasanian-Parthian Confederacy and the Arab Conquest of Iran, published in 2008, provides both a detailed overview of the problematic nature of trying to establish exactly what happened, and a great deal of original research that questions fundamental facts of the traditional narrative, including the timeline and specific dates. Pourshariati's central thesis is that contrary to what was commonly assumed, the Sassanian Empire was highly decentralized, and was in fact a "confederation" with the Parthians, who themselves retained a high level of independence. Despite their recent victories over the Byzantine Empire, the Parthians unexpectedly withdrew from the confederation, and the Sassanians were thus ill-prepared and ill-equipped to mount an effective and cohesive defense against the Muslim armies. Moreover, the powerful northern and eastern Parthian families, the kust-i khwarasan and kust-i adurbadagan, withdrew to their respective strongholds and made peace with the Arabs, refusing to fight alongside the Sassanians. Another important theme of Pourshariati's study is a re-evaluation of the traditional timeline. Pourshariati argues that the Arab conquest of Mesopotamia "took place, not, as has been conventionally believed, in the years 632–634, after the accession of the last Sasanian king Yazdgerd III (632–651) to power, but in the period from 628 to 632." An important consequence of this