I have read a couple more chapters in this book — which is thick with detail. I cannot honestly remember all the names of people and places. But what I take away is that the expansion of Islam was facilitated by unanticipated factors — such as the weakness of Rome, the devastation of the plague, etc.
Also I find fascinating how far Christianity had spread by the mid 630s. At one point, Kennedy asserts that most of the people of the Sasanian Empire were Christian and only the ruling elite practiced Zoroastrianism, which was the state religion.