The USA could Invade Pakistan but the problems are here compared to both Afghan and Iraq, Pakistani population is 10'x bigger 180 million the iraqi and afghan population combined is 44 mill, the USA had difficultly trying to to get the Insurgency in Iraq under control from a few thousand militants so an Insurgency in Pakistan would be Worse then Vietnam, I've been to Karachi, Lahore, Islamabad and several other places and it's not hard to get a gun, next the Pakistani army is far better then Iraqi army and most of the middle east, invasion would unite the Whole country, it took about 250,000 US troops to hold Iraq, the Idea of Holding Pakistan would be around 3 million or more, not to mention both China and Iran would aid Pakistan, Invading Pakistan yes the USA could but it would be stupid the lives lost would make it hell on earth.