Yup! You’re absolutely right. Independent movement had gathered massive force and I’d like to believe the Brits left abruptly to avoid a bloodbath. The opposite happened as you know. Partition was beyond horrendous. Though my family didn’t have to migrate from India, the stories they told of massacres are awful. Boundaries of India Pakistan drawn by the British are ludicrous, as you say, done in a hurry by a petty bureaucrat. East and west Pakistan separated by swathes of India! Ludicrous and unsustainable. Bangladesh was bound to break away. That too was a bloodbath full of atrocities perpetrated by Pakistan army on its own citizens.
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