You are quite right that tribalism is endemic to human cultural history, much as it is for chimpanzees and other apes. As E.O. Wilson often observed, tribalism is an innate tendency in all social animals, including ourselves. And this means, of course, seeing the "other" as a threat and perpetual on-and-off warfare. So I agree with you on this.

The difference, however, is that monotheism in particular--Judaic, Christian, and Muslim--took this kind of tribalism to a higher, more lethal level by asserting that there was only one true god, and that they were "his people" while all who opposed "his people" were therefore the enemies of the One True God as well. This means that compromise is tantamount to heresy, and bloodshed is the only possible way of dealing with the "enemies of god." We see this, of course, in the perpetual carnage in Israel/Palestine, as well as the perennial hatreds among various Muslim and Christian sects toward one another. It's all tied into the toxic idea that "WE, not you, are God's Chosen People.

Polytheistic cultures, such as ancient Greece and Rome, were far more tolerant of foreign cultures when they encountered them. Even when they conquered other peoples, both Hellenistic Greeks and later, Romans, left their subjects' temples and holy shrines intact, and were keenly curious about their rites and practices--Herodotus is a wonderful example. Conversely, once Theodosius made Christianity the official state religion of Rome around 391 AD, the Christians simply destroyed ALL the holy places of other sects, often erecting their own churches on the sites. They simply regarded all the others' deities as devils, and massacred anyone who adhered to the old practices.

In Tibet, on the other hand, diifferent schools of Buddhism, including the pre-Buddhist Bon culture, coexisted for centuries, often exchanging teachers and getting into debates and discussions with each other. And in China, Taoism, Confucianism, and Buddhism coexisted for centuries as well; lots of folks practiced an amalgam of all three.