The permanent question raised by the Iraq war is whether and when liberal imperialism is justified. After all, the war proponents were right that Ba'ath Iraq was an unimaginably brutal tyranny. Imagine Ontario run by Bernardo and Homolka. The realist and security-based argument for invading Iraq was always, to my mind, transparently weak. Once the inspectors were in there, and finding nothing, it was without any conceivable merit. But I couldn't dismiss the case for externally imposed armed liberation so easily.
Will Wilkinson has an interesting post in which he spells out the dilemma. Liberalism is rare in the world. It is also a good thing, at least in comparison to the nihilistic tyrannies that seem to be the alternative nowadays.
We cannot be certain how much of this is because of deep-rooted cultural factors, which are resistant to external change, and how much is due to the illiberal countries not obtaining a threshold of widespread non-resource-based wealth. I find both hypotheses plausible, and as a good Anglican (OK, a bad Anglican) imagine the truth is somewhere in the middle.
Wilkinson's dilemma can be stated as follows: to the extent that illiberalism is deep-rooted culturally, external military force will be ineffective at bringing about liberalism; conversely, to the extent illiberalism is the result of a lack of (the right sort) of development, external military force is unnecessary (and, because necessarily internally-generated-wealth-destroying, counter-productive) to bringing about liberalism. Either way, imperial liberalism doesn't work.
If imperial liberalism was prepared to be strictly Singapore-style or Raj-style authoritarian, and internally-generated liberalism was the product of economic development, then this argument might not work. The imperial power wouldn't allow elections, but would guarantee incorruptible justice and free markets.
The difficulty is that nationalist and democratic aspirations of the conquered people would necessarily manifest themselves in armed resistance to the imperial liberal regime. This would have to be met with illiberal force. The occupying army would have trouble distinguishing between resisting and non-resisting members of the conquered people, and, let's face it, would have no motivation to do so. Frankly, the distinction would be dubious -- there might be some who would collaborate with the occupation for personal advantage, but no one is ever going to side with occupiers because they are liberal. As E.O. Wilson, expert on ants, said about communism, "Good theory -- wrong species."
So, in the end, liberal imperialism is never going to work. It is especially not going to work when the members of the imperial army have a recent tribal grievance against the people they are occupying.
It is still possible that economic development will ultimately make the Middle East a more liberal place. It is also possible that nothing will ever do that. But what is never going to happen is that liberalism will come as a present of the US Marine Corps.