How India, Pakistan and Bangladesh were formed

In August 1947, the British decided to end their 200-year long rule in the Indian subcontinent and to divide it into two separate nations, Muslim-majority Pakistan and Hindu-majority India

The process of partition, however, was not simple. In addition to the British-controlled territories, the subcontinent also consisted of many other territories under French, Portuguese or Omani rule, as well as more than 500 sovereign princely states ruled by local monarchs.

Upon independence, the British gave the princely states the option to join India or Pakistan - by signing the Instrument of Accession - or to remain independent. Some of these territories and princely states did not become part of India or Pakistan until recently.

Today, Kashmir remains the only region of British India that has not been integrated into one of the two nations or gained independence.

Indian nationalist leader Jawaharlal Nehru (l), Viceroy of India Lord Louis Mountbatten (c) and the president of the All-India Muslim League Muhammad Ali Jinnah (r) discuss Partition in 1947 
 

Partition triggered one of the great calamities of the modern era, perhaps the biggest movement of people - outside war and famine - that the world has ever seen.

No one knows the precise numbers, but about 12 million people became refugees as they sought desperately to move from one newly independent nation to another.

The term partition of India does not cover the secession of Bangladesh from Pakistan in 1971, nor the earlier separations of Burma (now Myanmar) and Ceylon (now Sri Lanka) from the administration of British India

 The term also does not cover the political integration of Princely States into the two new dominions, nor the disputes of annexation or division arising in the princely states of Hyderabad,Junagadh and Jammu And Kashmir, though violence along religious lines did break out in some princely states at the time of the partition.

 It does not cover the incorporation of the enclaves of French India into India during the period 1947–1954, nor the annexation of  Goa and other districts of Portuguese India by India in 1961

How were the India-Pakistan partition borders drawn?

In July 1947, about five weeks before the British were scheduled to depart the Indian subcontinent, Sir Cyril Radcliffe, a British lawyer, was commissioned to draw the borders that would divide British India into two countries - Muslim-majority Pakistan and Hindu-majority India

This was Radcliffe's first-ever trip to India. He was asked to base his lines on the population of Muslims and Hindus, in addition to "other factors". These additional factors were never officially defined, but are believed to include economic and communication resources, such as irrigation channels and railway lines.

The Radcliffe Line was officially announced on August 17, 1947, a few days after the independence of India and Pakistan. The newly demarcated borders resulted into one of the biggest human migrations in modern history, with roughly 14 million people displaced. More than one million people were killed.