The Bangladesh War of Independence, or simply the Liberation War in Bangladesh, was a revolution and armed conflict sparked by the rise of the Bengali nationalist and self-determination movement in what was then East Pakistan during the 1971 Bangladesh genocide. It resulted in the independence of the People's Republic of Bangladesh. The war began after the Pakistani military junta based in West Pakistan launched Operation Searchlight against the people of East Pakistan on the night of 25 March 1971.
During the Bangladesh Liberation War of 1971, USSR played a key role by assisting Indiaupporto highlight the atrocities committed by Pakistan Army against the revolutionaries and protesters. Indira Gandhi, the Indian Prime Minister, extended military support to the leades of the national movement. The US, on the other hand, supported Pakistan via diplomacy and threat. Both President Nixon and Henry Kissinger were apprehensive of Soviet influence in the south Asia. The US sent military supplies to Pakistan and negotiated with China to curtail the Soviet dominance. President Nixon sent USS Enterprise, the aircraft carrier to the Bay of Bengal as a threat diplomacy to curtail Indian move while India took it as a nuclear threat. Both the US and USSR were engaged in the spread of their sphere of influence in Asia and hence they got involved indirectly in Bagladesh Liberation War.
USA and China come together to help Pakistan after they lose war against Indian Army. USA and China both having a fear of Indian dominance in the Soviet Union and its friendly relations with Russia because Russia always helps India at that time. To send warning signals to India to close down their dominance in western Pakistan and put India under pressure to limit down its relationship with Russia. Another common reason to help Pakistan by USA is there supply of weapons to Pakistan and generate good amount of revenue for there economy. After Bangladesh got independence, the US extended its support by helping Bangladesh with ~$300. However, during the liberation war, U.S supported Pakistan for several reasons,
1.India signed a mutual defense agreement with the then USSR, and India was helping Bangladesh to get independence. The agreement between USSR and India shook the U.S and hence, it supported Pakistan with weapons.
2. Upcoming Afghanistan war was yet another reason. USSR was interested in Afghanistan, and therefore, U.S made Pakistan an ally to help win war.