I've always hated the "if we knew then what we know now" rationale for opposing the war in Iraq. The problem that all of us who were right about the war had with it wasn't that it would necessarily go poorly - although that was also on the table, what with the outside force invading and reshaping a society through no desire of their own - but that the general idea of using bogus information as an impetus to preemptively invade a country was a bad idea on its own merits, even if it ended up with us sitting on piles of gold and barrels of oil as we high-fived Ahmed Chalabi at the new Chuck-E-Cheese in the middle of Tikrit.
Jeff Jacoby illustrates why. You see, if you oppose the Iraq War because it's going poorly, rather than because it was a bad idea on its own merits, you're then forced to deal with the routine outbreaks of, "Look! Progress!" that tend to infect the rhetoric of war supporters.
Not to again abuse World War II (at least it's not another Call of Duty game!), but going to war against the Axis was the right thing to do because we had the right reason to go - war was declared on us, and the fate of the free world was on the line. You go to war because it's the right and necessary thing to do in order to protect yourself and others from unjust harm. You don't go because it's going to go well, feel good and be profitable. That, you see, is imperialism.