The new Bush-Blair-Howard-Koizumi rules for waging war deserve attention. They say you are free to use whatever justification you like that if you want to attack someone.
If the justification proves false -- harboring terrorists and weapons of mass destruction, or WMD, in the case of Iraq -- you simply say that they might have existed, so what's the difference. If the other side allows inspections to prove the justification is false, you say why bother, since the attack is going to occur anyway.
Our forefathers would have appreciated the delightful simplicity of it all. They spent decades creating the now-outdated organizations and rules that were supposed to prevent or at least regulate warfare between nations. To enforce those rules they had to insist that the leaders of nations who declared war without valid justification should be strung up by their necks. Now all this is gone.