After learning about the general history of ancient Eurasia, I’ve a strong suspicion that the conflict in the Ambattha sutta is not between castes but between cultures. That is, the Sakyans were Central Asians and the Brahmins were from the settled “civilized” Indian culture. Central Asians were despised as barbarians by all the settled civilizations (Rome, Persia, India, and China) because they were nomadic, warlike, and egalitarian (women were more equal and leaders of confederations were elected by clan chiefs). A quick way to get a picture of what the Central Asian nomads were like, think “Klingons” from Star Trek the Next Generation.
By contrast, the settled peoples were authoritarian and peace-loving. They had hereditary kings, strict laws and customs, and women and minorities were repressed. They fought wars to build empires and establish a peaceful and orderly society. The nomadic peoples were a constant threat to that peace and order. And the settled civilizations considered them culturally repugnant. They didn’t build cities, didn’t bathe, didn’t read or write (in later eras), and most importantly, they were quick to settle differences with violence and behaved like “land pirates” raiding and exhorting wealth from the weaker settled peoples.
It’s a strange juxtaposition to us in modern times, but in ancient times authoritarian government was civilized and egalitarianism was barbaric. The world was divided between the civilized empires and nomadic free tribes that terrorized them periodically. The nomads were better at warfare than the civilized peoples because they had the best horses, skilled riders, excellent archers, and (most importantly) could be absolutely brutal. Even Roman legions, perhaps the best armies of the civilized world, found it very difficult to fight them to a draw. Atilla the Hun can be credited with toppling the western Roman empire even though he died before he could conquer them. He had done enough damage that they fell apart afterward and the Dark Ages began in Europe.
So, when I read the DN 3 Ambattha Sutta, I can see the conversation being one in which a so-called civilized priest encounters a barbarian shaman and treats him in this prejudiced way. It explains the brahmin reaction to encountering the Sakyan culture when he and his teacher visited one of their cities. “They were so uncouth!” Barbarians didn’t observe strict rules of politeness than urban peoples did, nor did they care much about castes or even gender roles. Women could divorce their husbands and talk back to them! It’s the same reaction we read about when a Chinese literati or a Roman diplomat encountered a Central Asian tribe.
(There’s a funny story of a Chinese diplomat who was captured by a Manchurian tribe and made to marry one of their women. He eventually escaped and returned to China and wrote the only description that still exists about that tribe’s culture at the time. But historians have to read between the lines of the Chinese author’s distain for those nasty barbarians who let their women behave like men!)
These stories like the one we read in DN 3 I think hint at the Sakyas being originally a Central Asian tribe that settled on the periphery of civilized India in modern-day Nepal. In the Dirgha Agama parallel DA 20, it says that when King Okkāka banished his sons, they went south to settle by the Himalayas. DN 3 doesn’t have that detail in that part of the story but later the Buddha does say,
He went to a southern country and memorized the Prime Spell. Then he approached King Okkāka and asked for the hand of his daughter Maddarūpī.
Meaning to me that the native land where King Okkaka was located somewhere north of India where his sons had settled and become the Sakya.
The implications of this are interesting. It may be the conflict between the ascetics and the brahmins in India during the Buddha’s time was between the religion of Central Asian shamans and the religion that had developed in the settle cities of India, with its castes, gods, and Vedas, etc. The two religious traditions were mixing, having conflicts, but gradually merging into a shared tradition that today we call Hinduism.