On November 21, 1971, Indian forces and Bangladeshi resistance fighters of the Mukti Bahini decisively defeated a Pakistani infantry and armoured attack during the two-day Battle of Garibpur (located in present-day Bangladesh). The Pakistanis lost 180 troops killed, 14 tanks destroyed or captured, and 3 planes downed or damaged, while Indian/Bangladeshi casualties were only 40 wounded. The aerial combat between the Indian and Pakistani air forces during this battle is known as the Battle of Boyra, where 2 Pakistani planes were shot down, and another was damaged but managed to land to safety.
(Taken from Bangladesh War of Independence and Indian-Pakistani War of 1971 – Wars of the 20th Century – Volume 3)
Since its independence, India had fought two wars against Pakistan and faced the perennial threat of fighting against or being attacked simultaneously from East Pakistan and West Pakistan. India therefore saw that the crisis in East Pakistan yielded one benefit – if the threat from East Pakistan was eliminated, India would not have to face the threat of a war on two fronts. Thus, just two days into the uprising in East Pakistan, India began to secretly support the independence of Bangladesh. The Indian Army covertly trained, armed, and funded the East Pakistani rebels, which within a few months, grew to a force of 100,000 fighters.
In May 1975, India finalized preparations for an invasion of East Pakistan, but moved the date of the operation to later in the year when the Himalayan border passes were inaccessible to a possible attack by the Chinese Army. India had been defeated by China in the 1962 Sino-Indian War, and thus was wary of Chinese intentions, more so since China and Pakistan maintained friendly relations and both considered India their common enemy. As a result, India entered into a defense treaty with the Soviet Union that guaranteed Soviet intervention in case India was attacked by a foreign power.
In late spring and summer of 1971, East Pakistani rebels based in West Bengal entered East Pakistan and carried out guerilla attacks against the Pakistani Army. These infiltration attacks included sabotaging military installations and attacking patrols, outposts, and other lightly defended army positions. Government forces threw back the attacks and sometimes entered into India in pursuit of the rebels.
By October 1971, the Indian Army became involved in the fighting, providing artillery support for rebel infiltrations and even openly engaging the Pakistani Army in medium-scale ground and air battles along the border areas near Garibpur and Boyra (Map 14).
India’s involvement in East Pakistan was condemned in West Pakistan, where war sentiment was running high by November 1971. On November 23, Pakistan declared a state of emergency and deployed large numbers of troops to the East Pakistani and West Pakistani borders with India. Then on December 3, 1971, Pakistani planes launched air strikes on air bases in India, particularly those in Jammu and Kashmir, Indian Punjab, and Haryana.
The next day, India declared war on Pakistan. India held a decisive military advantage, which would allow its armed forces to win the war in only 13 days. India had a 4:1 and 10:1 advantage over West Pakistan and East Pakistan, respectively, in terms of numbers of aircraft, allowing the Indians to gain mastery of the sky by the second day of the war.
India’s objective in the war was to achieve a rapid victory in East Pakistan before the UN imposed a ceasefire, and to hold off a possible Pakistani offensive from West Pakistan. In turn, Pakistan hoped to hold out in East Pakistan as long as possible, and to attack and make territorial gains in western India, which would allow the Pakistani government to negotiate in a superior position if the war went to mediation.
In the western sector of the war where opposing forces were more evenly matched, the fighting centered in three volatile areas: Jammu and Kashmir, Punjab, and Sindh-Rhajastan. Pakistan launched offensives that were generally unsuccessful, except in Chamb, a town in Kashmir which its forces overran and held temporarily. In the Longewal Desert in India’s Rajasthan State, a Pakistani armored thrust was thwarted by an Indian air attack, which resulted in heavy Pakistani losses.
In the Pakistani coast, Indian Navy ships attacked Karachi, Pakistan’s main port, and destroyed many Pakistani vessels as well as fuel storage facilities. Indian ships then blockaded the Bay of Bengal, cutting off East Pakistan from the sea and denying Pakistan maritime access to transfer troops and supplies from West Pakistan to East Pakistan.
The arduous topography of East Pakistan consisting of four major river systems and their thousands of smaller tributaries was thought by India to be the greatest natural obstacle for its armed forces to achieve a quick victory. With the support of East Pakistani fighters, the Indian Army therefore merely hoped to win as much territory as possible within a two-week period, and then allow Bangladeshis to install their government in the captured territory.