Visualizing the early spread of Arab Islam.

Historical Evolution of the (Western) Roman Empire

Source: Wikipedia Fall of the Western Roman Empire

Historical Evolution of the Byzantine (or Eastern Roman) Empire

Source: Wikipedia Decline of the Byzantine Empire

This may seem obvious, but I was just struck by the fact that the expansion of the Arabs and the related early expansion of Islam was related to the decline of Rome.

In 632, the year Mohammed died, the Arabian peninsula was united and Muslim.

The Roman empire was at its largest extent around the end of the 1st century CE. In the beginning of the 5th century CE the Roman empire was divided into a Western and an Eastern empire, and by the end of that century the Western empire had fallen.

The Sassanian Empire which arose in Persia in the 3rd century CE expanded until it was in conflict with Rome. There was a destructive Byzantine-Sassinid war from 602 to 628 which weakened both sides, and a civil war in the Sassanian Empire in the following decade. I assume that the Arab conquest of the Sassanian Empire from 632 to 634 was facilitated by the weakening of the Sassanians. So too, the conquest of Egypt by the Arabs would have been facilitated by the weakening of the Byzantine Empire. The spread of Arab control to the west in north Africa and into the Spanish peninsula would have been relatively easy in the turmoil after the fall of the Western Roman Empire, and indeed some of that turmoil had preceded the fall and helped lead to it.