Bangladesh (East-Pakistan) was part of Pakistan from 1947 to 1971.
The Government of India decided on 3rd December 1971, that India would go to war with Pakistan to save Bengali Muslims and Hindus.
On 16th December 1971, the chief of the Pakistani forces with 93,000 soldiers had surrendered unconditionally to the allied forces consisting of the Indian Army and Mukti Bahini in Dhaka.