In August 1947, British rule in India ended and the subcontinent was split into two states, Hindu-majority India and Muslim-majority Pakistan. The provinces of Punjab and Bengal were cut in two along religious lines, even though Hindus and Muslims had lived in mixed communities across them for generations.
The agreement didn't require anyone to move. People moved anyway, in fear, and violence followed them. About 15 million people were displaced in what historians call one of the largest forced migrations of the 20th century. The number of dead is still disputed. Scholarly estimates run from 200,000 to 2 million, and the Stanford historian Priya Satia gives half a million to 2 million.
Why so deadly? Satia points to the way Britain left. Officials, keen to keep both new states in the Commonwealth, hurried events at every turn. As the British Army departed and India's own army was being divided, confidential instructions in Punjab said British units had no operational role except to save British lives in an emergency. Police and bureaucracies broke down. Partition wasn't new to British policy either: it had already been used in Ireland and was being discussed for Palestine.
The lesson isn't that one side alone was to blame. Violence came from many hands. The lesson is how an empire's hasty, self-interested exit left millions unprotected.