Putting troops into pakistan would likely start world war 3. Besides the fact that they are an ALLY of sorts (UN and everything).... they are a nuclear state. Other countries in the region (probably most except for India and Bangladesh) would put us on their shit list for messing with them.

The main reason people faulted the US for going into Iraq was because we pretty much invented an imaginary reason to do so and the rest of the world saw our bullsh*t for what it was. Saddam didn't have any friends in the Gulf. The countries that didn't hate him (Kuwait, for obvious reasons) had a strong dislike and distrust for him (Iran, Saudi Arabia, etc).