In Peter Turchin’s work modeling human historical dynamics he introduces the idea of a “meta-ethnic” identity. Quite often this is synonymous with a world religion. These identities emerged in the last few years as human polities scaled so large as to expand beyond tribal-national boundaries.
These sorts of dynamics are clear when we think about the Crusades, the defense against the Ottomans in the 17th century, or the Iberian “division” of the world between Castile and Portugal. Common ties of civilization and identity allow for ingroup cohesion, as well as heightening hostilities against outgroups.
Of course there many exceptions. When reading The Great Sea: A Human History of the Mediterranean I recall being struck by how southern Italian city-states like Amalfi opportunistically allied with Muslim pirates against other Christian powers. Similarly, during the Battle of Vienna Protestant Hungarians marched with the Ottomans against the broader Christian alliance which came to the aid of the Habsburgs.
These are two instances which show short term self-interest or necessity driving choices of group coalitions. Amalfi, like later Italian city-states, found it in their interest to do business with Muslims, even if it was to the detriment of their co-religionists. This did not mean they were no longer Christians. But in many instances they put that identity aside for their own gains. In the case of the Protestant Hungarians there’s was an alliance of necessity.
As recounted in Divided by the Faith the decades leading up to the Battle of Vienna the Hungarians experienced a concerted campaign of conversion and persecution of the part of the Habsburg monarchy in concert withe Roman Catholic Church. The Habsburg’s Austrian lands were brought back fully into Catholicism, as was most of Imperial Hungary. It is no coincidence that Hungarian Reformed Protestantism was strong in the east, which had been under Ottoman influence. The arrival of an expansive Austrian monarchy was an existential threat for them.
The flip side are cases where groups with the same civilizational identity engage in wars over resources or boundaries. The conflict between Ethiopia and Eritrea would certainly fit into this mold, and to some extent the Great War in the Congo which has flared for two decades now.
This sort of dynamic has been used to argue that Samuel P. Huntington’s The Clash of Civilizations and the Remaking of World Order is not a useful framework. But on the contrary what Turchin and colleagues have shown is that over the long run civilizational fissures tend to result in the most vicious and dehumanizing wars.