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It would have to fall under a few things:
First, I would try diplomacy - something Bush didn't do with Iraq.
Second the evidence would have to be strong as to those weapons being there. In Pakistan, it is pretty certain that they have got them, they are where North Korea got their nuclear technology from.
In Iraq there was no such certainty.
Third, there would have to be a real risk of catastrophe involved - Pakistan and India have been engaged in an outright war for a long time now.
Iraq and its neighbours had been at peace for a fairly long time.
Fourth, there I would have to be convinced as to the level of threat posed by the religious extremists, and that they are indeed religious extremists.
Saddam, as bad as he was, was not a religious extremist and the threat he posed previously, had been neutred.
Fifth I would require UN sanction to do it. If the UN says no, back off the evidence isn't good enough.
If all five conditions are met, then I would support an effeciently fought invasion followed by installing an American general as the head of the country in question in order to rebuild the place. I would not bullshit people by claiming that Democracy comes with a single election.