The Indo-Pakistani War of 1971 was a devastating defeat for Pakistan. The war sped up the loss of Bangladesh, nearly collapsed the economy, and essentially goaded India and Pakistan into entering the nuclear arms race.
When 10 million refugees from the newly independent Bangladesh fled to India, great strain was put on the Indian government. India requested aid from the U.S. to deal with the situation, but U.S. president Nixon refused (Pakistan, meanwhile, enjoyed regular aid from the U.S.).
Empowered by this perceived political advantage, Pakistan initiated a particularly unprovoked attack against Indian airfields in Kashmir on December 3, 1971. India retaliated violently, and quickly gained the upper hand in the war, attacking and East and West Pakistan, taking some 90,000 prisoners of war. India’s stunningly swift victory was largely due to backing from the Soviet Union, to which India had turned given America’s ties with Pakistan.