So Jackson Diehl learned nothing from the Iraq war:

The problem here is not that advocates of the Iraq invasion have failed to learn its lessons. It is that opponents of that war, starting with Obama, have learned the wrong ones.

One lesson from Iraq that many war opponents have learned is that the U.S. shouldn’t be waging unnecessary wars that serve no discernible U.S. interest. That isn’t the wrong lesson to learn. It’s one that Diehl simply ignores, which is probably why he never really addresses how it would serve U.S. interests to go to war in Syria. Another lesson is that forcibly collapsing a regime creates far more instability and chaos than leaving it in place would. There is no likely scenario in which hastening regime collapse would have limited the loss of life and displacement of civilians in Syria. After all, the collapse of a regime doesn’t mean that fighting between different communities will stop, so even an intervention that felled the regime wouldn’t necessarily halt the conflict or limit the suffering of the civilian population. Sooner or later, the same people who said that the U.S. wouldn’t have to take responsibility for stabilizing a post-Assad Syria would be insisting that the U.S. do just that.

Everything in Diehl’s argument ultimately hinges on the assumption that U.S. military intervention in Syria would have been limited, easy, and effective:

As in the Balkans — or Libya — the limited use of U.S. airpower and collaboration with forces on the ground could have quickly put an end to the Assad regime 18 months ago, preventing 60,000 deaths and rise of al-Qaeda. It could still save the larger region from ruin.

It is doubtful that this would be quick, and it is also doubtful that this could be achieved by a “limited use” of airpower. Comparisons with Balkan interventions conveniently omit many of the factors that made those interventions “work” as well as they did. Even the Libyan war took eight months, and that was against a regime that was considerably less powerful and much more isolated internationally. Military interventions almost always take longer than expected, and that’s always true for interventions that are sold to the public by emphasizing how low-cost and easy they will be. Take an interventionist’s original estimate for how long a given military action will take, and then multiply it by ten or fifteen and you’ll be closer to the real figure. One of the reasons no one trusts the promises of Iraq war hawks is that they were promising a swift and easy war in 2003, too, and all that many of them can say after an eight-year debacle is that it “hasn’t turned out, so far, as we war supporters hoped.”

As for Diehl’s account of the costs of Iraq, it is not surprising that even now he understates the number of Iraqis driven out of their country and displaced inside it, as well as reducing the overall fiscal costs of the war to “hundreds of billions” instead of the trillions that it will be over the long term. One obvious lesson that everyone should have learned from Iraq is that hawks are always unduly optimistic about the cost, duration, and difficulty of waging war, and they are always far too confident in the efficacy of hard power, which causes them to believe that military action is advisable far more often than it really is. When it concerns starting an international war, which is what Diehl is proposing that the U.S. do, it is far better to err on the side of caution.