This kid covers the reasons and progression of how Bangladesh came about in a short paper:
http://www.astronomy.ohio-state.edu/~nahar/cv/bdliberation-essay8.pdf
Bangladesh Liberation War, 1971 By: Alburuj Razzaq Rahman 9 th Grade, Metro High School, Columbus, Ohio
The Bangladesh Liberation War in 1971 was for independence from Pakistan. India and Pakistan got independence from the British rule in 1947. Pakistan was formed for the Muslims and India had a majority of Hindus. Pakistan had two parts, East and West, which were separated by about 1,000 miles. East Pakistan was mainly the eastern part of the province of Bengal. The capital of Pakistan was Karachi in West Pakistan and was moved to Islamabad in 1958. However, due to discrimination in economy and ruling powers against them, the East Pakistanis vigorously protested and declared independence on March 26, 1971 under the leadership of Sheikh Mujibur Rahman. But during the year prior to that, to suppress the unrest in East Pakistan, the Pakistani government sent troops to East Pakistan and unleashed a massacre. And thus, the war for liberation commenced.
The Reasons for war :
Both East and West Pakistan remained united because of their religion, Islam. West Pakistan had 97% Muslims and East Pakistanis had 85% Muslims. However, there were several significant reasons that caused the East Pakistani people to fight for their independence.
West Pakistan had four provinces: Punjab, Sindh, Balochistan, and the North-West Frontier. The fifth province was East Pakistan. Having control over the provinces, the West used up more resources than the East. Between 1948 and 1960, East Pakistan made 70% of all of Pakistan's exports, while it only received 25% of imported money. In 1948, East Pakistan had 11 fabric mills while the West had nine. In 1971, the number of fabric mills in the West grew to 150 while the number in the East went down to 26. About 2.6 billion dollars of resources were also shifted over time from East Pakistan to West Pakistan.
Although East Pakistan had the largest population among all the provinces, it had much less political power than West Pakistan. This eventually made the people of East Pakistan rebel. Sheik Mujibur Rahman, the leader of the Awami League in East Pakistan, explicitly demanded more economic and political powers. The struggle finally culminated into the war of independence.
There was also the language issue that kept East Pakistan and West Pakistan in an uneasy status. In 1948, Mohammad Ali Jinnah stated in Dhaka that Urdu was the official language for Pakistan. There was a big argument about this because only the Muhajir in the West and the Biharis in the East spoke Urdu. Most of the West Pakistanis spoke Punjabi and Sindhi, while East Pakistanis spoke Bangla. East Pakistan therefore disagreed; seven students were killed in a fierce protest on February 21, 1952. This day has been remembered since then and is observed each year to emphasize the importance of the Bengali language. February 21st is now recognized as the International Mother Language Day by the United Nations.
A devastating cyclone hit East Pakistan in 1970. It was called the Bhola Cyclone. It killed about 500,000 people and made many more homeless. It brought great shock and deep depression among the East Pakistani people. But, the government did not provide enough relief to alleviate the extremely miserable conditions wrought by the cyclone. This caused enormous misery in East Pakistan.
The War :
The Awami League, led by Sheikh Mujibur Rahman, won a landslide victory in the national elections in 1971 and demanded autonomy for East Pakistan. The party won a 160 seats and a majority in the national assembly. This victory also gave it the right to form a government, but Zulfikar Ali Bhutto, the Chairman of the Pakistan People’s Party refused to let the Sheikh become the Prime Minister of Pakistan. This initiated the war. The Sheik gave a speech on March 7, 1971 when he urged the people to turn all their homes into a fort of fight. He demanded transfer of power to the elected representative before the assembly meeting on March 25.
Tikka Khan, a West Pakistani general, flew to Dhaka to become the Governor of East Bengal. But, the East Pakistani judges denied him entry. Thereafter, on the night of March 25, the Pakistani army tried to violently crush the Bengali’s opposition. Residence halls of the Dhaka University were viciously attacked. On March 26, the Pakistani forces arrested Sheikh Mujibur Rahman. On the same day, he signed an official declaration for the independence of Bangladesh. M. A. Hannan, an Awami league leader, is said to have been the first person to read and announce the Declaration of Independence over the radio.
Political events approached a climax. The war between the Pakistan Army and the Bengali freedom fighters, the Mukti Bahini, began. The head of the Mukti Bahini was General Muhammad Osmani. The Mukti Bahini were trained like guerillas. India gave shelter to the refugees and trained the Mukti Bahini. India also helped with ammunition and its own soldiers. They attacked the Pakistani army. During the training period of the Mukti Bahini, the Pakistani Army encouraged Razakars, the Bengalis who did not want Bangladesh to become an independent country, to suppress the rebellion. The Pakistani Army faced problems as the monsoon came. This helped Mukti Bahini because they could counter the moves of the Pakistanis.
India assumed an active role. Indira Gandhi ordered air and ground attacks. India, having superior equipment and forces, mounted a three-pronged movement on Dhaka from the Indian province West Bengal, Assam, and Tripura. The Indian soldiers, Air Force, and Navy defeated the Pakistani army, while the Bangladeshi Navy helped India. On the ground, three groups of Mukti Bahini and Indian forces fought the Pakistanis. The Pakistanis tried to fight back, but failed to resist them.
The Victory:
On December 16th, 1971, Dhaka fell to the Mitro Bahini, the elite forces of the Mukti Bahini and the Indian army. An “Instrument of Surrender” was signed by the defeated Pakistani General Niazi and by the Indian commander General Aurora at 16:31 Indian Standard Time. This is how Bangladesh became liberated and independent. December 16th is recognized as the Victory Day in Bangladesh, while March 26 is recognized as the Independence Day. With sovereignty, Bangladesh is progressing in all aspects.