AHC: make the political divide in the mediterrenean east-west, not south-north

so in OTL, the main political, economical and philosophical divide in the Mediterranean was set between south and north, and (arguably) caused by the expansion of Islam to encompass most of the southern Mediterranean.

What would make the divide of the Mediterranean east against west instead of north against south?
 
Islam does never exist or it doesn't spread out of Arabia, so "Orthodox" christianity remains as the major religion in Egypt, Anatolia, Cyrenaica and the Levant. The rest of the Mediterranean would be catholic.
 
Perhaps a failed Roman Empire results in an eastern Mediterranean Greek Thalassocracy, and a Carthaginian Thalassocracy in the west. While these empires would almost certainly not last to the modern day, they could lay the foundations for a cultural, religious, and linguistic divide which persists as migrants to the eastern and western Med are assimilated to the local cultures.
 
Perhaps a failed Roman Empire results in an eastern Mediterranean Greek Thalassocracy, and a Carthaginian Thalassocracy in the west. While these empires would almost certainly not last to the modern day, they could lay the foundations for a cultural, religious, and linguistic divide which persists as migrants to the eastern and western Med are assimilated to the local cultures.
I wonder how western Europe changes with their shaping figures being the Carthaginians, not Romans - The greeks always affected the eastern med. more, but the romans shaped the entirty of the western med. until their fall, and the northwestern med. until today. A punic Italy, Maghreb, Iberia and France sounds absolutely fascinating.
 
so in OTL, the main political, economical and philosophical divide in the Mediterranean was set between south and north, and (arguably) caused by the expansion of Islam to encompass most of the southern Mediterranean.

What would make the divide of the Mediterranean east against west instead of north against south?

Have Islam fail to penetrate further west than Libya. IOTL it took them several goes to finally conquer Carthage, and if you can keep that area in Christian hands, the western Med pretty much remains a Christian lake. In the east, the forces of Islam take over Anatolia and the southern Balkans, either in the first wave of conquests or later as IOTL. Hey presto, you now have a Mediterranean Sea where the main religious and cultural divide is between the west and east.
 
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