Excerpts from Books and Wikipedia
"The Battle of Yarmouk in 636 resulted in a crushing defeat for the larger Byzantine army; within three years, the Levant had been lost again. Heraclius died of an illness on 11 February 641; and most of Egypt had fallen by that time as well." [Wikipedia] "Succession crisis at Constantinople; accession of Constans II (641-668); Arab conquest of Egypt ; beginning of the Byzantine Dark age (641-867)" [Harl: World of Byzantium Guidebook, p. 117]