There are three distinct elements to consider here:
- the start of open all-out war
- cross-border operations by Indian forces in support of the Mukti Bahini
- material support and training provided to the Mukti Bahini by India
The Pakistani perception of a probable Indian involvement in the political developments of early 1971 is yet another interesting element which I plan to come back to in a future post.
First, on the start of open war, from Sarmila Bose's "Dead Reckoning":
The date of the start of full-fledged war between India and Pakistan in 1971 is a contested issue. The date popularly given out is 3 December, the one announced by India, but this is merely the date the war spread to include the Western sector. In a sense India's involvement in the war may be taken to be from March, and its involvement in the politics of the province perhaps from even earlier. Numerous Bangladeshi pro-liberation accounts blithely recount close contact and coordination with Indian authorities prior to the military action taken by the Pakistani regime, as well as Indian involvement and casualties in 'actions' in East Pakistan throughout the year. Many of the Pakistani officers I spoke to described Indian penetration of the territory as pervasive. 'The big operations are always done by the Indians', reported The Guardian on 18 September 1971, after an ethnic Bengali, who blended in with the local population and needed no translation, visited the training camps of the Mukti Bahini in India and crossed into East Pakistan with a guide on his own. Of the couple of hundred Bengali 'volunteers' who were said to be in the border area he visited, only six had been given any training at all and only three had taken part in any operation.
The start-date of the open all-out war in East Pakistan turns out not to have been 3 December after all. General Niazi, the Eastern Commander of the Pakistan army, was irritated enough by claims of a 'lightning campaign' by India to devote a separate section in his book to the subject, entitled 'The Date of the War': 'On the night of 20/21 November 1971, the Indian Army attacked East Pakistan from all directions'. General Niazi is of course an interested party in this debate, but his assertion is supported by the work of the American scholars Sisson and Rose. They conclude that India decided in favour of eventual direct military intervention as early as April 1971, and then devised a phased strategy. "The American government was correct in its assessment that India had already decided to launch a military operation in East Pakistan when Mrs. Gandhi came to Washington in early November pretending that she was still seeking a peaceful solution'.
Roughly speaking then, all-out war can be said to have started at the end of November, while large scale cross-border Indian operations took place as early as September. What about training and material support? We saw above that Sisson & Rose conclude that the Indian administration decided on military intervention as early as April 1971. Here is what they have to say about India's support for the Mukti Bahini:
By late July the decision-making group around Mrs. Gandhi had achieved a broad consensus on the issue of Bangladesh, laying the foundations for the domestic and foreign policy of the government thereafter. One of the more immediate consequences was the perceived need to establish more direct supervision of the Bangladesh government in exile and the Mukti Bahini and other Bangladeshi "liberation forces." The Indian army had assumed primary responsibility for the arming and training of the Mukti Bahini forces from the Border Security Force on 30 April. Even occasional participation by Indian military personnel in raids across the border was now permitted. An effort was also made to bring the different autonomous resistance groups under Mukti Bahini (that is, Indian) supervision and direction, but with only limited success.
The first effort to create an organized armed force from among these groups was made on 14 April by the Bangladesh government in exile when it appointed Col. M. A. G. Osmani, a military adviser to Mujib and a retired Pakistani army officer, as commander in chief of the Bangladeshi armed forces. Osmani had entered the army prior to independence and had been instrumental in raising and forging the East Bengal Rifles. He was a well-trained senior officer who had had high-level staff, but limited command, experience, and was reported to have felt that his career had been thwarted in the Pakistani army. He had not achieved the rank held by the Indian officers with whom he had to deal. Osmani was instrumental in creating a series of training camps along the Indo-Pakistani border in late April and May, first as centers to receive and temporarily house recruits and armed personnel coming from East Pakistan, but subsequently to impart training in small-unit tactics and in the use of firearms and explosives. Initial training was given by East Pakistani personnel in collaboration with personnel from India's Border Security Force (BSF) established after the 1965 war for policing functions, like the East Pakistan Rifles on the Pakistani side. The BSF was under police command, but included many retired Indian army personnel in its ranks. As the ranks of the Mukti Bahini recruits swelled, however, the Indian army assumed training and organizing functions. By the time serious hostilities commenced in November, Pakistani intelligence reported fifty-nine training camps in operation around the perimeter of East Pakistan. By the end of May a command structure had been created that incorporated a substantial proportion of the Bengali freedom fighters into company units, ultimately organized into three brigades responsible for guerrilla operations, with East Pakistan divided into various sectors, each under the charge of Bengali officers.
According to Pakistani intelligence, the Indian army started a more concerted effort to train Mukti Bahini personnel in June. Three hundred recruits were reportedly sent to Cochin for training as underwater saboteurs, and another three hundred, almost all university students, were trained at Plassy on the Bhagirathi River in West Bengal as frogmen. Student trainees were sorted in terms of their education. Science graduates were given two months' technical training; undergraduates were trained in small arms, mortars, recoilless rifles, rocket launchers, map reading, and commando tactics; non-matrics were trained as saboteurs in the use of explosives, mines, and grenades. Others were trained at artillery-and signal-training centers in Lucknow and Dehra Dun or selected for short courses in officer training in Dehra Dun. By the end of June some thirty thousand Mukti Bahini recruits had been trained, although in a rather slipshod manner; by November seventy thousand were under arms, with another thirty thousand reportedly undergoing training. According to both Pakistani and Indian sources, the weapons made available to the Mukti Bahini by India were limited and largely obsolete. Modern weapons became available when the Mukti Bahini could purchase them on the international and West Bengali arms markets. Indian political and military leaders considered it prudent to provide some training and weapons to guerrilla activities in East Pakistan; otherwise, the Mukti Bahini would have had to become directly involved in West Bengal in their search for arms, something that New Delhi preferred to discourage. They had to be kept involved in East Pakistan regardless of their military effectiveness.
Having concluded that India would eventually have to resort to the direct use of force against Pakistan, New Delhi set about devising a strategy that would achieve its objectives on the most cost-effective terms. A decision had been made as early as April to give some assistance to the Bangladeshi resistance forces as a form of pressure upon the Pakistani government to make basic concessions, but by midsummer it was clear that this would not be sufficient. The newly organized Mukti Bahini had not been able to prevent the Pakistani army from regaining control over all the major urban centers on the East Pakistani-Indian border and even establishing a tenuous authority in most of the rural areas. The next phase in Indian tactics, from July to mid October, involved both much more intensive training of the Mukti Bahini and direct involvement in Mukti Bahini activities by Indian military personnel. This enabled the Mukti Bahini to launch major organizational and sabotage campaigns in East Pakistan at the height of the monsoon season, at a time when the Pakistani army's maneuverability was reduced because of its dependence on vehicles and armored personnel carriers. The Mukti Bahini campaign, with some disguised Indian involvement, was directed, and with some success, at such strategic facilities as bridges, power stations, communication systems, and ships in Chittagong harbor. The destruction of bridges handicapped the Pakistani forces in this period, but it proved to be a major obstacle to the Indian invasion forces in December. Indian artillery stationed on the border was used on occasion to support Mukti Bahini activities in the immediate transborder areas, usually in response to Pakistani army shellings and incursions if the Indian reports are taken seriously.
The next phase in the preliminaries to open and direct intervention by Indian forces was the period from mid October to 20 November, in which both quantitative and qualitative changes occurred in Indian military support of the Mukti Bahini, particularly in several key strategic border areas. Indian artillery was used much more extensively in support of rebel operations in East Pakistan, and Indian military forces, including tanks and air power on a few occasions, were also used to back up the Mukti Bahini. Indian units were withdrawn to Indian territory once their objectives had been brought under the control of the Mukti Bahini, though at times this was only for short periods, as, to the irritation of the Indians, the Mukti Bahini forces rarely held their ground when the Pakistani army launched a counterattack. Nevertheless, quite substantial, if scattered, areas of East Pakistan had been brought under tenuous Mukti Bahini control by mid November, since the Pakistanis had to concentrate their limited forces on those sections of the border that were considered strategically critical.