I think Paksitan will use a nuclear weapon if majority of it's air force was knocked out, it's ordanance factories have been destroyed, it has lost several strategic land positions and it's ground forces are being bombarded. That is called a worst case scenario. However, I doubt it will come to that any time soon, as the stakes are far too high for any such conflict. Pakistan has influence in many Arab countries, along with the backing of China, so it will not be easy to wage a war against such odds unless someone is willing to start WW-III. However if both countries are fighting at about the same level and neither has any decisive advantage over the other, the war will continue, and probably end, without resorting to nuclear weapons. If India maintains air-superiority and Pakistan maintains ground dominance, it will be condisered an equal match. Nuclear weapons are used in desperation, as the final card in the deck, not like an artillery shell. And when one side does use a WMD, the other side will, by default, get the clear for also using a similar weapon. So it's a lose-lose for both sides, unless one side has nothing more to lose.
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