The India-Pakistan War 1971

Though this war is commonly known as Indo-Pakistan war, it is actually the Bangladesh Liberation War. This war started internally in Pakistan on 3rd December 1971 and ended with a major defeat to Pakistan on 16th December 1971 when Bangladesh gains its Independence.
The conflict was triggered by the unrest between the East and West Pakistan when the former demanded more freedom and autonomy. The East Pakistan constituted mostly of Bengalis. USA actively supported Pakistan in this war while India with support from France and United Kingdom backed the East Pakistani Bengalis in their fight for freedom. The coup de grace came when USSR joined India through a treaty signed on August 9th, 1972 which assured their friendship and cooperation at all levels between the two countries.
India launched a counter attack on Pakistan following their air assault on Indian Territory. The Indian Army was able to occupy the entire eastern half, which later declared itself as an Independent country, i.e. Bangladesh on December 6th, 1971. The internal conflict as well as the casualty of the war coupled with the fact that some 10 millions Bengalis fled the country to India (due the war) crippled Pakistan to a very large extent. For a while it was believed that Pakistan might not survive this defeat.
However, in 1972 India and Pakistan entered a new treaty called the Shimla Accord under the persuasion from the UN and peace once again prevailed. As a sign of goodwill, India returned a large part of the occupied territory – some 13 thousand kilometers square as a gesture of goodwill. In 1974 Pakistan recognized Bangladesh as an independent entity and country.