Not a historian, but here is a brief answer.
1970: A general election happened under military command. Among the 300 seat parliament, the Awami League got 160 ( 150 were needed to form a majority government) . While they did not won any seat in the west, 99% of the seats in the east pakistan belonged to them.
January 1971: Military government postponed the result as they refused to give away the power to Awami League.
March 1971: The discussion about how to proceed despite the clear election result failed. Awami League wanted to establish a USA style federal government to ensure the rights of the east. But the postponement of the parliament were seem as the West does not care at all about the east and would rather subjugate them. Mujib called people to start the struggle for independence on March 7. On March 25, pakistani army were unleased on civilians. They killed a lot of students at the major university in Dhaka, neutralized the provincial police force, and arrested any politicians whoever they could get their hands on . A radio transmission was broadcasted on behalf of Mujib declaring the independence. March 25-26, had the east-pakistani members in the military revolting, and one of them got hold of a better radio station and declared independence again.
April 10: The political leaders who were able to escape under the nose of military, regrouped in India. They travelled back to one of the border region inside Bangladesh where the Pakistani Army was not yet in control (ironically, it was near the place where the last independent king of bengal lost the battle against the british forces)
April-November 1971: Bangladesh army was formed formally consisting of regular ex-pakistani army members who escaped in march and volunteers who escaped to india to fight in the war. The indian army helped with arms and ammo. The bangladeshi army continued operation for the following 8 months. The other force made mostly out of volunteers were called Mukti Bahini ( freedom force ) who operated in the Viet Kong style (https://en.wikipedia.org/wiki/Mukti_Bahini)
Dec 1971: India refused a direct involvement to help Bangladeshi government in fear of triggering a backlash from international level. But their golden opportunity came when Pakistani army attacked them directly on their western border. India moved in quickly, cut of the supply line between east and west, entered Bangladesh alongside mukti bahini and bangladeshi army. They surrounded Dhaka in 15 december, bluffed the Pakistani command about handing them over to Mukti Bahini, which got them scared. Thus the open surrender happened in december 16.
This is how the liberation war can be seen as a civil war.