So you're asking what would happen if India, at some point, were to decides to replicate the Japanese attack on Pearl Harbour, assuming the USA and UK are collectively foolish enough to sell India a fleet of ships, aircraft, munitions, along with the training, over a few decades, to enable them to overpower the US Pacific Airforce, Pacific fleet, and perform a successful amphibious assault, say with a few more US supplied INS Jalashwa (L41)
class boats, on the US military base known as Camp Justice (Diego Garcia)
, in the British Indian ocean territory.
I suspect a phone call will be made between Washington and London and a coin tossed to decide whose trident nuclear subs, in Indian Ocean, will nuke which of the old colonial cities. The US Congress and UK Parliament will likely be called together for an emergency session, and an hour or two later: New Delhi, Bombay, Calcutta, Madras, Bangalore, … will have been reduced to ground level, and every Police / Military / Reservist body in the the US and UK summoned and sent off to round up anyone they suspect of being Indian. China, Pakistan and Nepal will then likely decide to permanently assist the survivors, and protect themselves, by deploying military aid units a hundred or so km's south of their current borders, and likely marking these permanent assistance zones with wire fences (Like Russia is still assisting a number of Japanese islands). While the rest of the sub-continent falls apart and starves, as the international community debate whether the action was proportionate, whether the populus should be assisted, how much aid would be required, how much it will cost, how it will be landed without the old colonial ports, who will donate, and who will skim what monies, and so on.
I'm surprised you didn't suggests the Indian army simultaneously launch a suprise attach on Aksai Chin, as a fight with 3 of the permanent 5 would obviously be better than just the 2.