Indian prime minister Indira Gandhi declared war on Pakistan and in aid of the Mukti Bahini, then ordered the immediate mobilisation of troops and launched the full-scale invasion. This marked the official start of the Indo-Pakistani War.
Three Indian corps were involved in the invasion of East Pakistan. They were supported by nearly three brigades of Mukti Bahini fighting alongside them, and many more fighting irregularly. This was far superior to the Pakistani army of three divisions[46]. The Indians quickly overran the country, bypassing heavily defended strongholds. Pakistani forces were unable to effectively counter the Indian attack, as they had been deployed in small units around the border to counter guerrilla attacks by the Mukti Bahini.[47] Unable to defend Dhaka, the Pakistanis surrendered on 16 December1971.
The speed of the Indian strategy can be gauged by the fact that one of the regiments of Indian army (7 Punjab now 8 Mechnised Inf Regiment) fought the liberation war along the Jessore and Khulna axis. They were newly converted to a mechanised regiment and it took them just 1 week to reach Khulna after capturing Jessore. Their losses were limited to just 2 newly acquired APCs (SKOT) from the Russians.
Indian Armys T-55 tanks on their way to Dhaka.
Indias military intervention played a crucial role in turning the tide in favour of the Bangladeshi rebels. Indias external intelligence agency, the R&AW, played a crucial role in providing logistic support to the Mukti Bahini during the initial stages of the war. R&AWs operations, in then-East Pakistan, was the largest covert operation in the history of South Asia.
Civil War | War 1971 Bangladesh