“In this inquiry I shall unfold the events that rescued our ancestors of Britain, and our neighbours of Gaul, from the civil and religious yoke of the Koran; that protected the majesty of Rome, and delayed the servitude of Constantinople; that invigorated the defence of the Christians, and scattered among their enemies the seeds of division and decay.”

The Decline And Fall Of The Roman Empire
by Edward Gibbon

I will always remember the first time I read this. It was early on in my research of the Apocalypse, Islam, and the Byzantine Empire and I was not sure where this was leading me but at that moment in time a historian of the eighteenth century told me I was on the right track.

Gibbon's footnote on the date is very interesting. His date 668 is at least five years earlier than most historians and I think he is still two years off

see 'Gibbon and the calculator'