India can target the Indus, pretty much gutting Pakistan. As long as they disrupt/poison the river enough they wouldn't have to target any settlements directly. If Pakistan developed the capacity to effectively target the Ganges (no small feat, and probably something best done with chemical weapons, not nuclear weapons), it would be possible to cause the displacement of a billion people over a couple of years. That would not be a good time to live in China, Europe, or Africa.

Of course, neither India nor Pakistan would want either of these things to happen. While India would survive an unilateral strike, Pakistan would not -- regardless of who struck the strike.