Perhaps the time was simply right? The Byzantines and Persians had spent most of the 6th and the early part of the 7th century merrily slaughtering one another. The result?
The issue is were they "united" by Islam then or somewhat later when a ruler decided to emulate Constantine?
http://en.wikipedia.org/wiki/Byzantine%E...sanid_Wars
Quote:The devastating impact of this last war, added to the cumulative effects of a century of almost continuous conflict, left both empires crippled. When Kavadh II died only months after coming to the throne, Persia was plunged into several years of dynastic turmoil and civil war. The Sassanids were further weakened by economic decline, heavy taxation from Khosrau II's campaigns, religious unrest, rigid social stratification, and the increasing power of the provincial landholders. The Roman Empire was even more severely affected, with its financial reserves exhausted by the war, the Balkans now largely in the hands of the Slavs, Anatolia devastated by repeated Persian invasions, and the empire's hold on Syria, Mesopotamia, Palestine and Egypt loosened by many years of Persian occupation. Neither empire was given any chance to recover, as within a few years they were struck by the onslaught of the Arabs, newly united by Islam.
The issue is were they "united" by Islam then or somewhat later when a ruler decided to emulate Constantine?
http://en.wikipedia.org/wiki/Byzantine%E...sanid_Wars
