That is an assumption, we have never seen that happening or not happening, we have never seen two nuclear armed countries go to war. the problem with India-Pakistan scenario is extreme paranoia, extreme distrust, and absolute hate for each other and being next-door neighbours with very little reaction time, no one would be in a position to exercise restrain, it would be a matter of survival or death, who fires first and most would have a better chance of survival. That is why I am totally against our strategy/doctrine of using tactical nukes to stop invading armoured columns, because that would invite a response and that is a failed strategy. Once we decide we have to use tactical nukes we MUST be prepared to fire a fist salvo of a minimum of 200 strategic nukes on all their 1st line targets.
You don't slap the opponent and wait for his response to decide whether to punch or not. Once you decided to have a go start with a knee and then don't stop until he is down and out. War is not a game of chess anymore, it's all out till one is out for good.