Why was British India split into two countries in 1947?

British India was split because its two largest political movements could not agree on how to share power once the British left, and because Britain's last viceroy concluded that partition had become necessary for a quick transfer of power. On 14-15 August 1947 two new dominions, India and Pakistan, came into legal existence as self-governing countries. The division was set out in the Indian Independence Act of 1947, and it cut through two provinces, Bengal and the Punjab, by district-wide majorities of Muslims or non-Muslims.

The demand for a separate Muslim homeland had grown over the 1940s. In March 1940 the Muslim League's Lahore Resolution asked for areas with Muslim majorities to be grouped into independent states. In 1946 Britain's Cabinet Mission tried to keep India united through a loose federation. The Muslim League accepted the plan, but after Jawaharlal Nehru of the Congress rejected its grouping of provinces, it collapsed. Violence followed, including Calcutta in August 1946, where official accounts count about 4,000 dead.

Louis Mountbatten, the last viceroy, had been told to avoid partition, but the tense communal situation changed his mind. In June 1947 leaders of the Congress, the Muslim League and the Sikhs agreed to it, against the opposition of Mahatma Gandhi. The line of the border was then drawn by one British lawyer, Cyril Radcliffe, with just five weeks to decide.

The human cost was vast, and the killing was done by all communities, on both sides of the new border. The partition displaced between 12 and 20 million people along religious lines. Estimates of deaths range from 200,000, the figure used by Mountbatten's defenders, to 2,000,000; the historian Richard Symonds put the lowest estimate at half a million, and most scholars accept approximately a million. The violence left an atmosphere of hostility and suspicion between the two states.