The Indo-Pakistani conflict was sparked by the armed liberation struggle in East Pakistan between the dominant Bengalis and the multi-ethnic West Pakistanis over the right to govern and the constitution.:24 The political tensions between East Bengal and West Pakistan had its origin in the creation of Pakistan as a result of the partition of India by the United Kingdom in 1947; the popular language movement in 1950; mass riots in East Bengal in 1964; and the mass protests in 1969. These led to the resignation of President Ayub Khan, who invited army chief General Yahya Khan to take over the central government.:xxx The geographical distance between the eastern and western wings of Pakistan was vast; East Pakistan lay over 1,000 miles (1,600 km) away, which greatly hampered any attempt to integrate the Bengali and the Pakistani cultures.:13–14:xxi
To overcome the Bengali domination and prevent formation of the central government in Islamabad, the controversial One Unit program established the two wings of East and West Pakistan. West Pakistanis' opposition to these efforts made it difficult to effectively govern both wings.:xxx In 1969, President Yahya Khan announced the first general elections and disestablished the status of West Pakistan as a single province in 1970, in order to restore it to its original heterogeneous status comprising four provinces, as defined at the time of establishment of Pakistan in 1947. In addition, there were also religious and racial tensions between Bengalis and the multi-ethnic West Pakistanis, as Bengalis looked different from the dominant West Pakistanis.:24–25
The general elections, held in 1970, resulted in East Pakistan's Awami League gaining 167 out of 169 seats for the East Pakistan Legislative Assembly, and a near-absolute majority in the 313-seat National Assembly, while the vote in West Pakistan was mostly won by the socialist Pakistan Peoples Party.:686–687 The Awami League leader Sheikh Mujibur Rahman stressed his political position by presenting his Six Points and endorsing the Bengalis' right to govern.:xxx The League's election success caused many West Pakistanis to fear that it would allow the Bengalis to draft the constitution based on the six-points and liberalism.:xlv
To resolve the crisis, the Ahsan–Yaqub Mission was formed to provide recommendations, and its findings were met with favourable reviews from the Awami League, the Pakistan Peoples Party, and the Pakistan Muslim League as well as from President Yahya Khan.:109–110
Map shows Pakistan and East Pakistan. Distance between the two was 1,000 miles (1,600 km) of Indian territory
However, the mission was not supported by the elements in the National Security Council and was subsequently vetoed.:110 Zulfikar Ali Bhutto, the chairman of Pakistan Peoples Party, endorsed the veto and subsequently refused to yield the premiership of Pakistan to Sheikh Mujibur Rahman. The Awami League called for general strikes in the country.:110 President Yahya Khan postponed the inauguration of the National Assembly, causing a shattering disillusionment to the Awami League and their supporters throughout East Pakistan. In reaction, Sheikh Mujibur Rahman called for general strikes that eventually shutdown the government, and dissidents in the East began targeting the ethnic Bihari community, which had supported West Pakistan.
In early March 1971, approximately 300 Biharis were slaughtered in riots by Bengali mobs in Chittagong alone. The Government of Pakistan used the "Bihari massacre" to justify its deployment of the military in East Pakistan on 25 March, when it initiated its military crackdown. President Yahya Khan called on the military - which was overwhelmingly led by West Pakistanis - to suppress dissent in the East, after accepting the resignation of Lieutenant-General Yaqub Ali Khan, the chief of staff of the East-Pakistani military.
Mass arrests of dissidents began and, after several days of strikes and non-cooperation, the Pakistani military, led by Lieutenant-General Tikka Khan, cracked down on Dhaka on the night of 25 March 1971. The government outlawed the Awami League, which forced many of its members and sympathisers into refuge in Eastern India. Mujib was arrested on the night of 25/26 March 1971 at about 1:30 am (as per Radio Pakistan's news on 29 March 1971) and taken to West Pakistan. Operation Searchlight, followed by Operation Barisal, attempted to kill the intellectual elite of the east.
On 26 March 1971, Major Ziaur Rahman of Pakistan Army declared the independence of Bangladesh on behalf of Sheikh Mujibur Rahman.
In April, the exiled Awami League leaders formed a government-in-exile in Baidyanathtala of Meherpur. The East Pakistan Rifles and Bengali officers in Pakistan's army, navy, and marines, defected to the rebellion after taking refuge in different parts of India. The Bangladesh Force, namely the Mukti Bahini, consisting of Niyomito Bahini (Regular Force) and Oniyomito Bahini (Guerilla Force), was formed under the retired colonel Mohammad Ataul Gani Osmani.