If the invasion was justified on moral grounds (removing a repressive regime from power and installing a democracy), it should have been sold on that basis. Iraq was the same dictatorship before 09/11. Would the people of Britain and the US have supported the war plans if 09/11 had not occurred? The justification for the invasion was that Iraq was a threat to the US. Given that this view was not merely an error in judgment, but a deliberate distortion; I don't see how you could call the invasion morally justified. Your WWII example is a poor one. Regardless of what was done after the war, the reason the Allies went to war had nothing to do with removing repressive regimes and installing democracy. There was reason and opportunity to do that long before the war began.