The Pakistan/India situation is a most interesting one to consider. It doesn’t directly affect the US or a “close ally,” yet it’s opening the door for the nuclear genie again. It would be the first warhead detonated in aggression (even nuke tests designed to intimidate another country aren’t really “aggression”) since World War II. I think that the rest of the big nuclear powers would get together and immediately order them to stop, for varying reasons.
United States: “We have no interest in seeing World War III – unless we start it.” -Wade, from “Goldeneye.” The quote is humerous, but that doesn’t make the attitude wrong.
Russia: They may not be as much of a superpower anymore, but their reasons would probably be much the same as the US: “If anyone in the world nukes someone us, it’s gonna be us.”
China: They have a lot to lose, considering that border they share with India and all. A missplaced bomb would send up a nice nuclear cloud over the border. Besides, they want to be the biggest power in Asia. Two other countries having a nuclear war without them would be bad for their influence.
The UK and France would join in, being other members of the security council. They might not be as influencial as the first three, but they do indeed have nukes of their own. And, of course, England has large ties to India (not really good ones, though).
I suspect that India and Pakistan would suddenly become aware of the collective cold stare of the rest of the world, and the single warning of “Stop now.” The United States itself wouldn’t actually launch an invasion, but I suspect China would have no hesitation with a massive retaliation at whomever fired the next missile or launched the next bomber. Everyone sort of wins. The world orders them to stop but lets China do the dirty work, China gets a large bump in Asian influence (and saves its own territory from fallout), and the nuclear genie bottle is capped again. Really, all the big powers have to do is provide international pressure and tell China “Have fun if they don’t stop.”