From Wikipedia
The Partition of India was the partition of British India on the basis of religious demographics that led to the creation, on August 14, 1947 and August 15, 1947, respectively, of the sovereign states of the Dominion of Pakistan (later Islamic Republic of Pakistan and People's Republic of Bangladesh) and the Union of India (later Republic of India).
The partition was promulgated in the Indian Independence Act 1947 and resulted in the dissolution of the British Indian Empire. The partition displaced up to 12.5 million people in the former British Indian Empire, with estimates of loss of life varying from several hundred thousand to a million.
The partition of India included the geographical division of the Bengal province of British India into East Pakistan and West Bengal (India), and the similar partition of the Punjab province into West Punjab (later Punjab (Pakistan) and Islamabad Capital Territory) and East Punjab (later Punjab (India), Haryana and Himachal Pradesh). The partition deal also included the division of state assets, including the British Indian Army, the Indian Civil Service and other administrative services, the Indian railways, and the central treasury.
In the aftermath of Partition, the princely states of India, which had been left by the Indian Independence Act 1947 to choose whether to accede to India or Pakistan or to remain outside them, were all incorporated into one or other of the new dominions. The question of the choice to be made in this connection by Jammu and Kashmir led to the Indo-Pakistani War of 1947 and other wars and conflicts between India and Pakistan.