Pakistan is mostly Muslim, India mostly Hindu. The two groups have been at war with each other for many centuries. If you have any friends or aquaintances who are Hindu, ask them about it. Islamic people may not know about it, but if they are Islamic and from anywhere near India (eg Pakistan) then they will have lots to say on the topic as well.

Even more recently, India and Pakistan were originally supposed to be one country. That was unworkable due to the history of hatred, so we got two countries. Ghandi was killed for trying to keep them from fighting. Since that failure they have had a series of wars and tense face-offs. The most scary from our perspective being the arms race in the late 90's that they had first for the nuclear bomb, then for the hydrogen bomb.

So yes, it is an extremely good bet that if there was a general offensive against Pakistan, India would join in to settle old grudges.

Ben