Manekshaw if he was ready to go to war with Pakistan. Manekshaw refused on the basis of some difficulties (including climatic conditions of the monsoon in East Pakistan) and he also offered to resign, an offer which Indira Gandhi declined. He then said he could guarantee victory if she would allow him to prepare for the conflict By the end of April 1971, Indira Gandhi asked Indian Army Chief Sam on his terms, and set a date for it. Indira Gandhi accepted his conditions.
By November, war seemed inevitable. Throughout November, thousands of people led by West Pakistani politicians marched in Lahore and across West Pakistan, calling for Pakistan to Crush India. India responded by starting a massive buildup of Indian forces on the borders. The Indian military waited until December, when the drier ground would make for easier operations andHimalayan passes would be closed by snow, preventing any Chinese intervention. On 23 November, Yahya Khan declared a state of emergency in all of Pakistan and told his people to prepare for war.
On the evening of 3 December, at about 5:40 pm, the Pakistani Air Force (PAF) launched a pre-emptive strike on eleven airfields in north-western India, including Agra, which was 300 miles (480 km) from the border. At the time of this attack the Taj Mahal was camouflaged with a forest of twigs and leaves and draped with burlap because its marble glowed like a white beacon in the moonlight.
This preemptive strike known as Operation Chengiz Khan, was inspired by the success of Israeli Operation Focus in the Arab–IsraeliSix Day War. But, unlike the Israeli attack on Arab airbases in 1967 which involved a large number of Israeli planes, Pakistan flew no more than 50 planes to India.
In an address to the nation on radio that same evening, Prime Minister Indira Gandhi held that the air strikes were a declaration of war against India ,and the Indian Air Force responded with initial air strikes that very night. These air strikes were expanded to massive retaliatory air strikes the next morning.
This marked the official start of the Indo-Pakistani War of 1971. Prime Minister Indira Gandhi ordered the immediate mobilisation of troops and launched a full-scale invasion. This involved Indian forces in a massive coordinated air, sea, and land assault. The main Indian objective on the Eastern front was to capture Dacca and on the western front was to prevent Pakistan from entering Indian soil. There was no Indian intention of conducting any major offensive into West Pakistan.
Bangladesh, 16 December 1971 