I'd like to think (diabolical choice of words) that evidence linking the 9/11 attacks directly back to the ISI would lend impetus to an invasion effort. But Pakistani intelligence has been very actively assisting the Taliban and AQ's campaigns in Afghanistan for close to a decade, and there's barely been a tepid U.S. response, so it's a
very tricky one indeed.
An invasion of Pakistan, whichever way you swing it, is going to be Iraq on bath salts. Who's going to offer themselves up as a launchpad? China won't. Iran definitely won't. India's setting itself up for its own funeral if it resigns to facilitate a U.S. war on its doorstep (I can actually see a really severe souring of Indo-American relations if it goes ahead). Anybody who has an inkling about the Afghanistan campaign will tell you an invasion through the Hindu Kush is logistically impossible - the virtual entirety of the U.S. Army's supply train runs straight through Pakistan, and Islamabad
has used this as a bargaining chip in the past. So you're going to have to look at a pre-September 11th POD to pull this off, and the operation's going to have to be seaborne.
Now, there's a really nasty MacGuffin we can't afford to forget about here. Pakistan has a large, relatively sophisticated nuclear weapons arsenal, consisting of around 100-130 warheads. Within a week of international forces going ashore, the Pakistani state, a mess as it already is, is going to hell. The CIA's No. 1 priority is going to be the minute documentation and capture of every nuke in the country. Here's the hitch: there's so many militant groups and so many political divisions, I can't see a situation where, at the very least, a handful of those weapons don't go missing. That means India is fucked, and the West is staring straight in the face the nightmarish prospect of Osama Bin Laden calling the shots on a real WMD.
Now, let's apply the final logistical nail to George W. Bush's neoconservative cranium. Iraq has a population of 33 million; it's distributed sparsely across arid flatlands and marshes, terrain that, in theory, shouldn't owe itself to guerrilla warfare. Pakistan's population is
182 million (and counting), so good luck stringing together an effective policing regime - the government itself struggles right now. As if the stakes hadn't been high enough, the ultra-traditional tribal areas (i.e. where foreign forces are going to face the brunt of resistance) are impenetrably mountainous. If IS and the Mahdi Army can pull off a protracted, unending insurgency in Iraq for well over a decade, it's going to look like a cakewalk in comparison to Pakistan.
In short, an invasion of Pakistan can only turn out mega-bad. Like, impossibly bad. I can't see Washington ever authorizing it.