Against a country that is multiethnic, with a severely alienated populace.
That's often overrated. Iran was multi-ethnic, with a severely alienated populace. Saddam counted on that, to his misfortune. Declare war, they'll all put aside their differences to fight you.
Furthermore, it's an actual war against a country, not propping up a tottering government against a determined insurgency,
Much, much, much harder to have an actual war against a country, rather than supporting a shaky country against an ongoing insurgency.
and chances are we will have the support of it's gigantic neighbor to the east.
Good luck with that. India's history has been one of dedicated neutrality. I'm sure that they would happily support America's war on Pakistan, in exchange for the massive numbers of refugees and the regional dislocations that will come out of that. I'm sure that India will have no concerns about destabilization from its own Muslim population. Nor about the repercussions for its relationships and conflicts with China and Russia.
That's just the beginning of it. Again, for this to feasibly happen, Pakistan has to do something incredibly stupid
You mean creating and supporting the Taliban? Giving shelter to Osama Bin Laden and his buddies and relatives? Trafficking in illegal nuclear weapons technology to countries like Iran or North Korea. Yeah, none of these things are likely to happen.
or has to be on the verge of going feral.
If the Pakistani state has collapsed... then why bother to invade?
That means the US has world support. Where is the Ho Chi Minh trail or superpower support for Pakistan?
You mean like the Coalition of the Billing?
It's not worth it.
But Pakistan doesn't have a chance of beating India in a conventional war in every war game simulation that has been done. They won't be beating the US.
Gravity, as the Tick said, is a harsh mistress.
If we want to bomb Pakistan, we can. We can pretty much bomb any country in the world with varying degrees of impunity. But there's a difference between just throwing a few jets and missiles around, and actually conquering an entire country. There's a world of difference between places like Grenada and Panama, or broken cripples like Afghanistan and Iraq and a place like Pakistan.
We could theoretically conquer the place, in the same sense that we could theoretically mount a manned expedition to Jupiter.
In practical terms, it would require more resources than we could sustainably muster.