The war broke out on 26 March 1971 as army units directed by West Pakistan launched a military operation in East Pakistan against Bengali civilians, students, intelligentsia, and armed personnel who were demanding separation of the East from West Pakistan. Bengali military, paramilitary, and civilians formed the Mukti Bahini and used guerrilla warfare tactics to fight against the West Pakistan army. India provided economic, military and diplomatic support to the Mukti Bahini rebels, leading Pakistan to launch Operation Chengiz Khan, a pre-emptive attack on the western border of India which started the Indo-Pakistani War of 1971.