Jon Chait is hard on himself too. He had the integrity to write a post on the invasion of Iraq (which he supported) on its 10th anniversary. This was clearly very hard for him to do. In fact, he didn't manage to stick to mie colpe and went on to arn anti-interventionists that they will have to apologise some time too.
Still it was a noble effort. Unfortunately, I think he still doesn't undestand why his 2003 reasoning was total nonsense. I tried to explain this to him in a comment.
It is admirable of you to so concisely explain the logic behind your support for the invasion. The logic however, is very strange. You seem to jump from an analysis of whether an invasion would be a violation of international law to the conclusion that it would be a good idea.
You note that you had an incorrect belief that there were WMD in Iraq. I also believed that with great confidence. I concluded that, therefore, it would be a terrible idea to invade. Imagine Iraq with anthrax and nerve gas. It is very clear that the WMD would not have been secured by the invading armies (we know that because conventional weapons weren't secured at all). some would have fallen into the hands of insurgents.
Are you seriously prepared to argue that it would have been unlikely for none of the WMD to end up in the hands of al Qaeda in Iraq ? I note the organization didn't exist when we invaded and that it has access to massive amounts of high explosives.
Did you really think it preferable to have WMD in a chaotic post war country(not inevitably as chaotic as it was but still chaotic) than in a country ruled by a deterable dictator ? I opposed the invasion, but doubted my judgment exactly when I found out that there weren't WMD in Iraq. Obviously, WMD in Iraq would have been an excellent reason to not invade Iraq. Here is the 12th blog post I wrote in my life making an argument which you still haven't faced
I have other criticisms too. I note your examples of successful interventions did not include invasion and occupation.
At the time of the invasion Blix was begging for more time -- for months not years not weeks months. Bush's huge rush should have convinced us that he feared Blix would report that Iraq was WMD fee. The evidence on WMD in Iraq changed in 2003 as Saddam Hussein ceased his interference in inspections (the Bush administration said active participation was required -- that is handing over WMD -- it is now clear that there was nothing Saddam Hussein could have done to convince Bush he had complied). In the event, you are clearly wrong about international law (the invasion was illegal as Saddam Hussein had complied). This isn't as important as understanding that something can be legal but not a good idea.
A problem is that it is very costly to admit that new evidence has proven one wrong. Professional incentives lead to rational stubborness.