In the case of Pk and India, departing colonizers got hold of a clueless cartographer
Who hurriedly and arbitrarily decided boundaries of Pakistan. Since bengal had a high concentration of Muslims and resistance was born there, that part of bengal became EAST Pakistan. Punjab, NWFP, Sind, Baluchistan belt that runs through Afghanistan was chopped away from Afghanistan. All with vastly different languages. East and west Pakistan had a huge swathe of India separating the two wings!
Even as they left, colonizers screwed us up royally. How could that ever work??
Civil war ensued and amidst much rape and brutality - civil war - East Pakistan became Bangladesh. Atrocities committed by Pakistan army in erstwhile East Pakistan are unconscionable.
In democratic elections Mr. mujeeb ur Rahman won. He should’ve been PM. But of course centrist Punjab didn’t want that and Bhutto took over as PM, having lost to Mujeeb. Naturally they seceded and today we have a flourishing Bangladesh while Pk is riddled with failures and boundless govt corruption.