The usual case against the Iraq War is that we initiated the war to remove "Weapons Of Mass Destruction" (WMDs) that post-war intelligence did not reveal, although Donald Rumsfield, Dick Cheney and a few others central to starting the war are convinced that the post-war search was just botched. (There was never any connection between 9-11 and Iraq either.)
Neuroskeptic takes a different tack on the issue. The concept of Weapons of Mass destruction is overinclusive, he argues. Nuclear bombs are without a doubt highly destructive. But, no one claimed that Iraq had or was about to obtain those. Yet, there is no good evidence that chemical weapons (which is what the Iraqis were allegedly developing and had used in the past), or biological weapons, are actually particularly worse than plain old bullets and conventional explosives delivered en masse, which have been equally deadly or worse than any past example of chemical or biological weapon useage. He does not even include the most extreme examples of killings with conventional weapons such as Dresden or World War I trench warfare. He acknowledges the theoretical mass destruction risk posed by biological weapons, which could cause a global plague, but Iraq wasn't alleged to have been developing those either.
Neuroskeptic's analysis is notable, because it denies that there was good cause to start the Iraq War even if everything that intelligence sources had alleged was true. As he points out, chemical weapons, even if Iraq had them, wouldn't be any different in the magnitude of the destruction that had the capacity to cause than their ample conventional weapon capacities.
Iraq and Afghanistan considered
The lack of a proper cause to go to war in Iraq, in turn, matters, because Iraq has not demonstrably been left better off than it was under Saddam Hussein.
Certainly, Hussein wasn't a nice guy. He was your run of the mill totalitarian Middle Eastern dictator. But, Iraq had a more developed and economically productive middle class than most of its neighbors; a large share of that middle class went into exile as a result of the war and the nation's infrastructure has still not recovered. The strict totalitarian rule of the regime made possible interethnic toleration that dissolved into ethnic cleansing after the invasion - ethnically mixed neighborhoods and regions have vanished in favor of strict ethnic segregation, the few ethnically mixed areas that remain are constantly at the brink of ethnic violence, and Iraqi Christians have pretty much been run out of the country.
Saddam Hussein was no worse that dicators like Egypt's Mubarak whom we supported heavily until shortly before his fall, and many other dictators and absolute monarchs whom we have supported around the world. Indeed, not so long before the Gulf War we did support Saddam Hussein as a bulwark against Iran and a guarantor of the flow of oil.
Iraq does have a civilian parliamentary government, made possible, in part, by reassurances of foreign military intervention on its behalf, but it is deeply disfunctional, particularly at the national level. It isn't entirely clear that there is a strong sentiment for continuing to operate as a single country. But for international disapproval, the Kurds would have left long ago, and the Shiite Southeast has secessionist leanings. Basic public safety is illusive, as suicide bombers in Iraq are happy to remind us. Is the average Iraqi better off? And, if not, did the war accomplishing anything worthwhile?
It also remains unclear what purpose was served by resurrecting Kuwait, a slave holding absolute monarchy with a tiny number of people keeping massive oil resources to themselves for the purpose of enhancing their standard of living to the detriment of their much more populous Iraqi neighbor, in the first Gulf War, after it had already been conquered by Iraq.
The no fly zone imposed to protect autonomous Kurdish region between the two Iraq Wars from human rights abuses instigated by their own totalitarian dictator is perhaps the only U.S. involvement in that time period that was really justified.
The Gulf War and Iraq War did demonstrate overwhelming Western military superiority over this Third World dicatorship. Indeed, they provoke a revolution in U.S. military strategy, tactics and weapons. But, one normally doesn't consider conducting realistic training exercises to be a sufficient justification for causing hundreds of thousands of people to die in war.
Afghanistan, was at least verifiably the base of the terrorists who caused 9-11, had arguably the most awful domestic government in the world under the Taliban, and, as I've noted before, was so profoundly squalid when we found it that it would be hard for anything we could do to make it worse. They had neither a semi-functional government nor any infrastructure of note to ruin, they were already in the midst of a bloody civil war that we muted for a number of years, the U.S. commitment has been an order of magnitude less than it was in Iraq, and the Afghani middle class went into exile more than two decades before we became involved. Indeed, given U.S. involvement in the resistance to the Russian occupation of Afghanistan, we were arguably partially responsible for the dismal state of affairs there and had some moral obligation to fix it. While the Afghan government is corrupt and insecure, it was created with more democratic involvement from the Afghans, is less vexed with deadlock, and is a more clear improvement relative to the Taliban or loose alliance of warlords that controlled respective territories in the country prior to U.S. intervention.
Moreover, the U.S. objective in Afghanistan to retaliate for 9-11 was accomplished, and the goal of making it unavailable as a terrorist base from which the U.S. can be attacked isn't a terribly ambitious one and doesn't require Afghanistan to become an example for the rest of the world to emulate. The extrajudicial abductions and torture tactics used in the Afghan War and War on Terror, however, may have done more harm than good to U.S. interests, inciting more to join the terrorist cause.
Our near simultaneous removal of Iran's two greatest external military threats may not have been in U.S. interests either, and there is a very plausible case that Iran really is or was trying to develop a true weapon of mass destruction, a nuclear bomb.
The one X factor that might arguably justify the Iraq War is the extent to which it has provided a proof of concept model for an Isalmic Arab democracy without resorting to theocracy, and by example, made the revolutions in Tunisia and elsewhere possible. But, given the sorry example that it did provide, it is hard to give this justification, post hoc, of course, much credence. What did Iraq's democracy show that the Kurdish and Afghan examples did not? If anything, it illustrated the limits of democratic government as a solution.