India, July 16 -- Like many villages across India, Karamchedu had two drinking water tanks - a sprawling one used by the dominant Kamma communities and a decrepit one used by the Dalit Madiga groups. For decades, this hierarchical compact had held because of a wicked cocktail of oppression and helplessness in a region where the division between the land-owning communities and the labourers was stark. The farmhands earned lower than the minimum wage and many were locked into generational cycles of debt by agricultural landlords.

On July 16, 1985, that compact broke.

That evening, a young Kamma man was washing his buffaloes near the steps of the tank used by the Dalits. A disabled Madiga man and another woman from the community objected t...