It'd be fair to say that the vast majority of conflicts have been infighting between groups in the same primary language families, ethnic descent, and religious groups, so that sort of solidarity hasn't been all too prominent for most of history. That's more an issue of proximity and competition for resources than anything else, I'd imagine, but Europeans Christians tended to fight other European Christians, whether for heresy or succession or any of the million other reasons people fight wars. Same applies for most regions of the world for the same reasons.Except in this ATL,they would ally. There would be two hypothetical explanations that would be fabricated. That Greeks and Persians have a common descent(we know that today) or that both have a same God in their beliefs. Such an alliance would help both survive for long.
For more specific examples, the Japanese fought l̶i̶t̶e̶r̶a̶l̶l̶y̶ ̶3̶ ̶w̶a̶r̶s̶ ̶l̶a̶s̶t̶i̶n̶g̶ ̶s̶o̶m̶e̶t̶h̶i̶n̶g̶ ̶l̶i̶k̶e̶ ̶2̶0̶ ̶y̶e̶a̶r̶s̶ ̶t̶o̶t̶a̶l̶ more like 7 major (Imjin War (7), Ryukyu Expedition (<1), 1st Sino-Japanese War (<1), Russo-Japanese War (1), Boxer Rebellion (1), 2nd Sino-Japanese War into WWII (8)) totaling less than 20 years with the non-Japanese in the past 700 years while fighting over 100 years against other Japanese in a brutal and extended civil war, the bloodiest war in US history in terms of American causalities was the US Civil War, the Italian Wars saw the Italians fighting each other, the French, the Spanish, Austrians, etc., the Slavs in the collapse of Yugoslavia are rather self-explanatory, the 30, 80, and 100 Years' Wars, etc.
Wars with groups of different ethnolinguistic groups, by comparison, are rather scarce as there tend to be fewer such neighbors nearby to declare war on and realpolitik demands pragmatism with regard to allies (the French and Turk against the Habsburgs, for example). And united efforts against even those neighbors tended to fall apart (the Ottoman advance failed to see Christiandom united; hell, part of the reason the Turks did so well was because the Christian nations of the Balkans had spent so much effort fighting each other they hadn't the power to resist Osman's onslaught and the rest of Europe continued to beat each other senseless for another couple centuries in the Italian Wars, Religious Wars (30 Years', 80 Years', French Wars of Religion), succession (Time of Troubles, the Byzantine civil war of 1341-1347 where one side allied with the Turks against their fellow Greeks, the extinction of the senior branch of the de Valois, etc.) before finally turning around to address the Ottomans in much of a meaningful manner).
The point being common descent and a similar religious background (the Abrahamic God and Zoroastrianism's Ahura Mazda aren't the same and Christianity and Zoroastrianism aren't even heresies of each other; the latter influenced Judaism, the precursor of the former, but they're not of the same religious fabric however you cut it) doesn't do squat to keep the peace between two entities, let alone see trust and cooperation between two traditional enemies; it hasn't historically and there's thousands of examples (civil wars, wars of religion against heretics, wars of conquest against neighbors, succession wars, wars of independence, wars to weaken enemies, pretty much every flavour of war) pointing to just that.
The age of sail and nationalism might've changed the ratio a bit, sure, but this is addressing the point of the Eastern Roman Empire and the Persians in the 7th century joining forces specifically for a common descent from ancestors that diverged thousands of years prior and rather distant linguistic and utterly tenuous religious connections rather than just because of the pragmatic desire to not both get overrun by a bunch of nomadic upstarts.
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