Fred Donner in the context of "Arab conquests"

The early Muslim conquests or early Islamic conquests (Arabic: الْفُتُوحَاتُ الإسْلَامِيَّة, romanized: al-Futūḥāt al-ʾIslāmiyya), also known as the Arab conquests, were a series of wars initiated in the 7th century by Muhammad, the prophet of Islam. He established the first Islamic state in Medina, Arabia that expanded rapidly under the Rashidun Caliphate and the Umayyad Caliphate, culminating in Muslim rule being established in Asia, Northern Africa, and Southern Europe over the following century. According to historian James Buchan: "In speed and extent, the first Arab conquests were matched only by those of Alexander the Great, and they were more lasting." At their height, the territory that was conquered by the Arab Muslims stretched from Iberia (at the Pyrenees) in the west to India (at Sind) in the east; Muslim control spanned Sicily, most of the Middle East and North Africa, and the Caucasus and Central Asia.

Among other drastic changes, the early Muslim conquests brought about the collapse of the Sasanian Empire and great territorial losses for the Byzantine Empire. Explanations for the Muslim victories have been difficult to discover, primarily because only fragmentary sources have survived from the period. American scholar Fred McGraw Donner suggests that Muhammad's establishment of an Islamic state in Arabia coupled with ideological (i.e., religious) coherence and mobilization constituted the main factor that propelled the early Muslim armies to successfully establish, in the timespan of roughly a century, one of the largest empires in history. Estimates of the total area of the combined territory held by the early Muslim polities at the conquests' peak have been as high as 13,000,000 square kilometres (5,000,000 sq mi). Most historians also agree that, as another primary factor determining the early Muslim conquests' success, the Sasanians and the Byzantines were militarily and economically exhausted from decades of warfare against each other.