The reason there is an Atmospheric Test Ban Treaty is because, shortly after the Soviet Union carried out one such test on the far side of the planet, unexpectedly, the radioactive isotope Strontium-90 started showing up in cow’s milk in Kansas.
The isotopes were picked up by the Jet Stream, carried around the world, and dropped through an outlet in the stream over the American Midwest. We immediately negotiated with the Soviets to ban further testing.
So what does this have to do with India and Pakistan?
South of both nations is a major ocean trading route, one of the larger ones in the world. China regards it as vital to their national security. They have gone so far as to build a major port in Pakistan, on condition their navy can use it as a base to protect their commercial shipping.
As such, a nuclear exchange between the two would force the Chinese to involve themselves. A third nuclear power much more powerful than the other two. Iran or Russia might want to get involved in it as well.
Scenarios like this are why the US takes such an active role in both nuclear non-proliferation and mediation. There are just way too many ways things can really get out of control and sooner or later trip us up as well.
And the effect of a nuke landing in NE Pakistan would be the end to China’s dream of a land route for trade with the Mideast.
The Mideast is becoming China’s problem.
And good riddance.