The Great Arab Conquests is a summary of the initial 100 years (approx) erruption of Islam from its founding heartland of Arabia, relating events on a territory by territory basis.
Kennedy begins discouragingly by setting out the problem faced by the historian: the lack of detailed and reliable contemporary record. Unfortunately, but predictably, this problem is not overcome and the success of the conquests ultimately remains an enigma.
At commencement the two regional superpowers were Byzantine and Persia. To the east the Arabs subsumed the whole of the Persian Sasanian empire and extended their dominion beyond as far as Sind. They took from the Byzantines the Fertile Crescent and northern Africa before conquering almost all of Spain and Portugal, and leaving a Byzantine rump corresponding to modern day Turkey, Greece and the Balkans.
Both empires had been ravaged in the 6th century by bubonic plague. At the turn of the 7th century they fought a ruinous war against each other leaving them further depleted economically and demographically. Trade in the mediterranean had partly collapsed due to the strife in the former Western Roman Empire. Great cities were left depopulated by this combination of circumstances. Religious divisions between Christians meant that local communities often felt little allegiance to Byzantine. Yet these factors alone do not explain why time after time Arab armies overcame substantially more numerous opponents. Ultimately Kennedy has no real explanation for this - a Muslim is left entitled to attribute it to God's will.
What is striking for the modern reader is that the primary purpose of the conquests does not appear to have been religous conversion, which usually occurred only gradually over the ensuing 200-300 years. Rather it was a process of military conquest. There was a strong economic imperative in the initial form of "booty" and subsequently by means of the poll tax that non-muslim peoples required to pay in order to live peacefully. Wealth flowed from the conquered lands to Damascus in the form of precious metals and stones, and in human form as slaves (the Berbers of north Africa suffering in particular). Relatively small Arab populations formed miltary and administrative elites in the conquered lands with life otherwise going on much as before for the local populations. It is difficult not to see a parallel with the British presence in India a thousand years later.
Kennedy tells his story in a simple narrative style with occassional humour but his prose is often flat. The territory by territory presentation has its drawbacks. It is often hard to relate simultaneous events in different geographic areas. The central policy of Damascus (if one existed) is hardly defined. The religous and political disputes within the central authority are alluded to but not well explained.