When the British came to India, Bengal was India’s richest province.
The British would back certain Indian princely states with financial and military resources over others, or alternatively install puppet rulers during rule disputes, then in return for their backing they would demand immense sums of revenue which the princely states would be unable to procure leading to increasing and basically unpayable debt—at which point the British would take over politically. When this happened in Bengal the British started using the tax revenues from Bengal to pay for Bengal’s own exports to Britain. Essentially a policy of total wealth extraction, draining Bengal’s resources and causing, previously unthinkable, famines that wiped out perhaps 50% of Bengal’s total population.
When the British left India, Bengal was India’s most impoverished province.