Its a little more complicated than that. India has never invaded pakistan, it has always been pakistan that initiated wards. Also, these days there is also the concept of pak sponsored terrorism on our soil, which is war by other means.
The choice to go nuclear is always yours. India will not use nukes first. India will not invade your territory either, unless you are stupid enough to try another kargil or mumbai.
If you think it is wise to use a nuke on your soil on our troops, go ahead and do it. Yes, one division or two will be vaporised, but the response would be karachi or lahore or islamabad getting vaporised. Are you sure you want to invite that by nuking a conventional invading force, or would you rather try and fight them man to man? The choice is yours, and always will be.
Mumbai wasn't state-sponsored and ISI actually tried to help India avoiding mumbai. So pul-eeeseee forget that any Mumbai,in which Pakistan is not even involved,would give India chance to strike Pakistan? Never.Period.
So again its YOUR choice...want to invade Pakistan? Cool...have no problems with that...
Also just for two divisions vaporized on ENEMY territory,you'd launch a nuclear strike on Pak? mmmm really? realistically? Well then be ready to see Mumbai , Hyderabad , Delhi , Chennai , Deccan , Kolkata etc etc etc getting vaporized into the air like fire's smoke. India would be a loser in such contest because in the nuclear war , "survivor" is the loser ! Pakistan won't be there no more but your GENERATIONS will SUFFER for eternity.
So again situation is simple :
Not mess with us , we won't mess with you.
Mess with us , we will see you in hell then .....
or would you rather try and fight them man to man? The choice is yours, and always will be.
Well offcourse we will fight them man to man. Its simple .. See
If Indians invade and Pakistani military is able to protect its key interests and defend itself against Indian armoured thrusts while inflicting heavy causalities on India (which would be obvious) then it would be India's defeat. No one would call it "punitive strike" ...
and IF (and a BIG IF) Indians over-whelm Pakistani forces and "punished" them then what you think? We'll let your forces go back ...home sweet home..? No sir this is not happening. In THAT case , Indian forces on OUR side would be liquidized and turn into smoke...
So again ....situations remains the same :
Not invade Pakistan , Pakistan won't do anything silly.
Invade Pakistan , death and destruction is waiting for the region.