The map below shows present-day India, Pakistan, and Bangladesh and how these countries were partitioned in 1947 as the colony of British India gained independence.
As you will see in the small box, today's Bangladesh was originally part of independent Pakistan, though it was culturally and geographically distinct. As part of Pakistan, East Pakistan was politically and economically marginalized from the central government in West Pakistan and experienced cultural, political, and economic oppression.
Movements for greater autonomy for the people of East Pakistan were met with bloody military crackdowns, eventually resulting in the 1971 genocide committed by the West Pakistan army against Bengali independence fighters and civilians.
This map appeared in The Economist on 13 August 2021. The Economist Explains piece that accompanied it is also reproduced in full below.