Let's assume that the first Arab siege of Constantinople in 674-678 succeeds and the Caliphate manages to control most of Anatolia, pushing the Byzantine Empire into collapse.
Considering that the consolidation of Umayyad power in Anatolia and Thracia would have been pretty challenging and demanding, it would probably mean that IOTL Umayyad conquest of Africa (690s) would have been postponed.
So, the Byzantines could have the chance of reorganize themselves in the surviving Western part of the Empire: Africa, Sicily and Southern Italy (maybe part of Greece too, even if now Greece is dangerously exposed). However it is quite obvious that in this scenario the Byzantines would realize that they alone could not stop a further expansion of the powerful Caliphate once it would be ready for another push.
In such scenario, Byzantines should try to form a solid alliance with other Christian nations in order to keep the Arabs at bay, but none of the main candidates (Visigoths, Merovingian Franks, Lombards...) seem to be powerful enough unless all or most of them participate.
Could an alliance of West Byzantines, Visigoths, Merovingians and other Christian Germanic nations be viable at the late 7th century in order to stop the advance of the Caliphate in the Euromediterranean region?
Considering that the consolidation of Umayyad power in Anatolia and Thracia would have been pretty challenging and demanding, it would probably mean that IOTL Umayyad conquest of Africa (690s) would have been postponed.
So, the Byzantines could have the chance of reorganize themselves in the surviving Western part of the Empire: Africa, Sicily and Southern Italy (maybe part of Greece too, even if now Greece is dangerously exposed). However it is quite obvious that in this scenario the Byzantines would realize that they alone could not stop a further expansion of the powerful Caliphate once it would be ready for another push.
In such scenario, Byzantines should try to form a solid alliance with other Christian nations in order to keep the Arabs at bay, but none of the main candidates (Visigoths, Merovingian Franks, Lombards...) seem to be powerful enough unless all or most of them participate.
Could an alliance of West Byzantines, Visigoths, Merovingians and other Christian Germanic nations be viable at the late 7th century in order to stop the advance of the Caliphate in the Euromediterranean region?