WI: Ghandi assassinated by a Muslim

why would hte Sikhs, Parsis, Untouchables, and other groups join the war against the Muslims?

The Parsis wouldn't start anything- they're a mercantile urban caste- but they'd get caught up in the violence. As for the Sikhs, well, they'd get caught up in it too.

Racial and religious conflict is messy, Keenir- it has a way of spiralling out of control. A squad of Sikh policemen gets sent in to restore order in a particular district which happens to be, say, Hindu. The cry goes up "The Sikhs are beating Hindus in the street". Then everything goes to hell in a handbasket. Pakistan and India would fall apart.

Same thing with any other group- this would be on such a big scale that everyone would get drawn in. Possibly the states in the far South like Kerala, Tamil Nadu and Karnataka might avoid the worst of the violence but everything from the Deccan to the Himalayas will go up in blood and fire.
 
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