Wary of the growing involvement of India, the Pakistan Air Force (PAF) launched a pre-emptive strike on Indian Air Force bases on 3 December 1971. The attack was modelled on the Israeli Air Force's Operation Focus during the Six-Day War, and intended to neutralise the Indian Air Force planes on the ground. The strike was seen by India as an open act of unprovoked aggression. This marked the official start of the Indo-Pakistani War.
As a response to the attack, both India and Pakistan formally acknowledged the "existence of a state of war between the two countries", even though neither government had formally issued a declaration of war.
The first shot fired by Indian armed forces when arriving into Bangladesh.
Three Indian corps were involved in the liberation 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. The Indians quickly overran the country, selectively engaging or 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. Unable to defend Dhaka, the Pakistanis surrendered on 16 December 1971.
The air and naval war
The Indian Air Force carried out several sorties against Pakistan, and within a week, IAF aircraft dominated the skies of East Pakistan. It achieved near-total air supremacy by the end of the first week as the entire Pakistani air contingent in the east, PAF No.14 Squadron, was grounded because of Indian and Bangladesh airstrikes at Tejgaon, Kurmitolla, Lal Munir Hat and Shamsher Nagar. Sea Hawks from INS Vikrant also struck Chittagong, Barisal and Cox's Bazar, destroying the eastern wing of the Pakistan Navy and effectively blockading the East Pakistan ports, thereby cutting off any escape routes for the stranded Pakistani soldiers. The nascent Bangladesh Navy (comprising officers and sailors who defected from the Pakistani Navy) aided the Indians in the marine warfare, carrying out attacks, most notably Operation Jackpot.