From Wikipedia

Partition - The Day India Burned

The Partition of India was the partition of British India 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 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), and also the division of other assets, including the British Indian Army, the Indian Civil Service and other administrative services, the Indian railways, and the central treasury. The partition was promulgated in the Indian Independence Act 1947 and resulted in the dissolution of the British Indian Empire.

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.

The secession of Bangladesh from Pakistan in 1971 is not covered by the term Partition of India, nor is the earlier separation of Burma from the administration of British India, or the earlier separation of Ceylon (now Sri Lanka). Ceylon, part of the Madras Presidency of British India from 1795 until 1798, became a separate Crown Colony in 1798. Burma, gradually annexed by the British during 1826–86 and governed as a part of the British Indian administration until 1937, was directly administered thereafter.  Burma was granted independence on January 4, 1948 and Ceylon on February 4, 1948. (See History of Sri Lanka and History of Burma) The Kingdom of Sikkim was established as a princely state after the Anglo-Sikkimese Treaty of 1861, however, the issue of sovereignty was left undefined. In 1947, Sikkim became an independent kingdom under the suzerainty of India and remained so until 1975 when it was absorbed into India as the 22nd state.

The remaining countries of present-day South Asia are Nepal, Bhutan, and the Maldives. The first two, Nepal and Bhutan, having signed treaties with the British designating them as independent states, were never a part of British India, and therefore their borders were not affected by the partition. The Maldives, which became a protectorate of the British crown in 1887 and gained its independence in 1965, was also unaffected by the partition.

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.