well its sham theory that they can invade Pakistan, they will never, yes worst scenario they can bomb Pakistan. with nukes in hand that will be very tough decision for US too.
there will be no all out war, they are already in huge mess in Iraq and Afghanistan, last thing they will want to have another enemy with very well trained and established Army not bunch of cow boys they face in Afghanistan or Iraq.
second option always an option, and Pakistan has capability to use the same tactics they have used against ruskies in Afghanistan, i dont think American want Pakistan on the other side. If Pakistan just stop co operating with US forces , they will be like lame ducks in Afghanistan.
I do not think that this situation would have come if Pakistan was under President Musharraf as it was during last many years. Fortunately, with his intelligent insight, we did not see any such incidence in past where Pakistan army was directly attacked by Afghan and USA forces, even when American operation is going on in Afghanistan since 2001. Many may not have liked his policy but it is better to be far sighted and calculative than fool hardy idiot. I hope that things get better as I would not like to see a single innocent Pakistani or people in Pakistan forces losing their life for pathetic Taliban, Al-Qauida and Afghanistan.
Anyhow, Pakistan in 80s did nothing against Russia other than played role of dog for USA. Job of Pakistan throughout that war was to support and feed ignorant and retarded Afghans so that they can do work for USA in the name of Islam. Fact is that Afghan '1980 war' was in the name of Islam but in reality it was work for Shaitan (Naam tha Islam ka per kaam tha shaitan ka). We have to understand that all those so-called Mujahadeens were recruited in the army of Shaitan because their retarded mind full of ignorance got influenced by propaganda by Shaitan (USA, Saudia and Zia government). Russia did not attack Pakistan because Russia knew that Pakistan was just a pawn doing job for USA and real power behind Afghan rebels was USA, so if Russia had attacked Pakistan then USA would have been there to defend and Russia was in no mood to start World War for Afghanistan.
This time there is no world power supporting Pakistan that can stand in between Pakistan and USA. Second is that, USA does not have to even enter Pakistan physically as they can simply bomb Pakistan to rubbles, especially Tribal area with millions of innocent casualties. Unfortunately, Pakistani forces are in no position to defend Pakistan or Tribal areas against really big and intentional USA bombings. If such would happen than only loser would be Pakistan and poor of Pakistan, as life in Pakistan would become miserable and unbearable, while life in country (USA) responsible for bombing would be normal as normal as when USA was bombing Iraq and Afghanistan.
Some Pakistanis sitting outside Pakistan would clap that Pakistan killed 10 Americans (of maybe Afghan, Arab or possible Pakistani origin) and at the same time would be making their point that USA is Zalim because they have killed million innocent Pakistanis. I do not believe that lives of Pakistanis are so cheap that it should be put on risk so that these retards could make their point or some misguided followers of Taliban and AL-Quaida could find their 'Journey to hell wish' come true.
Another thing USA would do to harm Pakistan would be to start supporting various separatist movements in Pakistan with arms, money, and international support and that itself would be disaster for Pakistan. Further, USA can arm India to teeth with let say 1000s of latest aircrafts, missiles, tanks, and other sophisticated military equipments and incite India to attack Pakistan taking Kashmir and breaking Pakistan into small units.
