The Partition of India was the partition of the British Indian Empire which 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).

This event was to result in the biggest mass migration in history. Over 18 million people migrated to join their particular religious majority. At least a million people died in communal violence in the process.

See:
http://en.wikipedia.org/wiki/Partition_of_India
The Hidden Story of Partition and its Legacies – BBC
The Road to Partition 1939-47 – National Archives
PAKISTAN OR THE PARTITION OF INDIA BY Dr. B. R. Ambedkar