OK, but that said, let's be economists about this. Let's do as Steyn purports (and in my opinion utterly fails) to do, and do a cold, calculating, rational cost-benefit analysis of the Iraq War.
This is very difficult to do, for two reasons. First, a counterfactual history is nearly impossible to construct; who knows how many of the things that have happened in connection with the Iraq War would have happened anyway, and who knows what other things would have happened? Second, the long-term effects of the war are not known; as with the French Revolution, it's "too early to tell" and always will be. But in these cases, you work with what you've got. So...
Costs of the Iraq War
* 150,000 - 200,000 Iraqis killed. This is a low estimate, but like many historians I tend to believe low estimates when it comes to war casualties, since there are a lot of refugees in wars, since government systems for counting people break down, and since a lot of people would have died anyway, especially given the crushing sanctions regime in place before the war. So let's make it 150-200k with an asterisk; it might have been three times that. (What's a factor of three between friends?)
* 4,400 Americans killed. Why list Americans separately from Iraqis (aren't they all humans)? Answer: Because many people care about this difference.
* 32,000 Americans wounded. This could range from a cut on the arm to four lost limbs. Due to improved battlefield medicine, there are more of the very severe "four lost limb" type injuries now than in past wars.
* Hundreds of thousands of Iraqis wounded. Probably. I'm not sure anyone has been able to count this.
* About 6 trillion dollars of U.S. money spent (about 40% of one year of U.S. GDP)
* A moderate, permanent loss of American prestige in Europe (possibly inevitable due to the end of the Cold War).
* A large temporary loss of American prestige in Europe, reversed when the Obama administration came into power.
* A moderate, permanent loss of American prestige in the Islamic world, from an already low level.
* A solidifying of the Russia-China alliance, which looks very capable of containing American power globally.
* A temporary distraction from the hunt for al Qaeda, reversed when Obama came into power and successfully killed al Qaeda's leadership.
* A long-lasting degradation of the quality of U.S. public discourse and the quality of U.S. politics. This item bears some explanation. In order to sell the war to the American people, large amounts of lying and distortion were necessary. Because of the stickiness of partisan opinions and worldviews (no one ever wants to admit their side was wrong), this meant that Republicans and conservatives had to retain that Bush-distorted worldview long after the war. That might be the reason why Tea Party types are up in arms about Benghazi, while the rest of America doesn't even know what Benghazi is.
* An increase in geopolitical strength for Iran, commonly believed to be a strategic enemy of the U.S.
* An acceleration of Iran's nuclear program, seen by Iran as the only deterrent that can prevent a U.S. invasion.
* A high continuing rate of violence in Iraq.
Benefits of the Iraq War
* The removal of the Hussein family from power in Iraq, and their replacement with marginally less effective, malign, and insane dictators.
* The elimination of the tiny, tiny risk that Saddam would one day develop WMDs.
* An improved Iraqi economy. Iraq's GDP, which had been crushed by sanctions, after the war recovered strongly, growing robustly in every year since 2006. Part of this, of course, is a windfall due to high oil prices; but if the Iraq War hadn't happened, sanctions might have prevented Iraq from selling a lot of its oil.
* An Iraq that is slightly more free than under Saddam. Iraq is still rated "Not Free" by Freedom House, though its ratings have improved ever so slightly since the war. Freedom House is a U.S. government-sponsored NGO, so it's not a good idea to trust their data implicitly, but this seems to agree with many other reports.
* A revitalized American liberal movement. The blogosphere as we know it really took off in response to the Iraq War, becoming the liberal answer to conservative talk radio. Truly liberal media outlets like MSNBC also emerged as answers to Fox News. And the general galvanization of America's dormant left might have enabled the election of Obama and sped the conservative retreat that we now see happening.
* The breaking of 9/11 fever. This is also hard to pin down, and may not even be real, but after 9/11 I felt a real sort of general madness in America. Terrorists could hit us any time, anywhere. The government was eliminating civil liberties right and left and people seemed to be fine with that. America seemed headed for a dark period of fear-based fascism, or Islamophobic ethnic conflict, or...well, something. Then the Iraq War came and brought more than half of America back to its senses. We remembered that the biggest threat to us is our own stupidity. We realized that the panic over a global jihadist wave was 99.9% paranoia. While the Republicans stayed nuts, the Democrats came back to the reality-based community.
Of course, these last two are a little silly to include as "benefits" of the war, since they weren't intended by the war's promoters (though neither were most of the costs). It's a bit like saying that the creation of the UN and the democratization of Europe were positive effects of Hitler's invasions, or that the worldwide condemnation of genocide was a positive result of the Holocaust. You should never start a war in the hope that you'll be defeated and that your defeat will invigorate the forces of good.
Anyway, so what do we conclude from this cost-benefit analysis? It's very hard to put dollar figures on these things, but I didn't try, because what this exercise should clearly demonstrate is that very, very little apparent benefit resulted from the decision to invade Iraq, while there were a whole lot of very apparent costs. This fact should dominate all discussions of the war. Who cares if we "won"? Who cares if the Surge (i.e. paying Sunni militias to stop bombing us, while pretending to make a show of force) worked? Who cares if Saddam was a brutal, awful guy?
What should matter is that we paid a lot, and we got not much. There are a lot of two-bit dictatorships we could randomly invade and depose in bloody wars. There would be benefits to getting rid of Kim Jong-Un, or Robert Mugabe, or the mullahs of Iran. They just wouldn't be worth the price tag.