Battle between the confredation of the tribes of Upper Sindh and the tribe of Bharata which lived east of the Sutlej (1400 BCE)
The main battle took place near the ravi river, with each side on the either side of the ravi. The Bharatas were just like the rest of the tribes were from somewhere geographically behind afghanistan. Orignally just like half the world, including europeans they were once the same people, that spoke Proto-Indo-European language but expanded and assimilated with the native populations and evolved culturally in seprate ways until they developed unique identities. When they entered the upper sindh plain the indus valley civilization was already in decline and is assumed to have become rural again. We do not know the scale of hostility between IVC and the invaders, but the rigveda (a hindu scripture) frequently uses harsh and derogatory terms against groups called Dasas and Dasyus.
However the incoming population assimilated and one of the cultures formed was the vedic culture. The hindu caste system that is still relevant now, roots from this culture. Similarly, just like today, perhaps because of geography itself, the sindh valley didnt keep the caste system. The bharatas used to insult the other tribes of the sindh plain for not following the caste system, among other thing such as eating meat and drinking wine and not following their customs. The alliance possibly included the persians who may have assisted, what they saw as distant kin. It also consisted several tribes that were part of bharata tribe but were in conflict with them. But it seems likely the most of the population and the tribes belong to a cultural branch that was somewhere in between just like dardic and nuristani languages, which is its own distinct branch that evolved from the previously mentioned proto-language.
According to vedic scriptures which name it the battle of 10 kings, bharata tribe won, and the confredation lost. However instead of expanding to the west they instead expanded to the east to the gangetic plains where vedic culture further developed. Vedic scriptures are religious scriptures not historical ones, so ofcourse they antagonize the enemies. For example they mention one of their god directly decimating 60,000 people. So, its facts should be take with a grain of salt, but it does set a general outline for what happened. Until we have more evidence, or texts from the confredation side we cannot know for reach any conclusion.
