One of the weakest argument in favor of going to war in Iraq was that we had to go get the terrorists who were operating inside Iraq. This argument was only plausible to people who were mistaken about what was happening in Iraq, which is why it was alluded to far more often than it was made explicitly. When claims about terrorists in Iraq were put into words, they tended to take a very limited form, specifying a few individual terrorists or even just one man, Abu Musab al-Zarqawi. A war to take down Baghdad was not a good way to stop Zarqawi, as was clear then and is all the more clear now. A far more promising approach would have been to go after Zarqawi directly, and his organization Ansar al-Islam, at their camp in the northern part of Iraq that was not even under the control of the Baghdad government.
Going to war in Iraq was not a way to go after the terrorists, as a chorus of people who opposed the war have tirelessly repeated: "Iraq is a diversion from the war on terror."
What response is there for those in favor of the war?
Here's one reply: "Yes, and a diversion is just what we needed." We wanted to divert the terrorists from their war on us. They're so distracted by the fight and the chaos in Iraq that they are neglecting their operations in other parts of the world. Because we're fighting them in Iraq, they aren't able to fight us nearly as effectively in other parts of the world.
Now, there are moral responses to this argument (how could you use the Iraqis as terrorist bait?!?), but let's consider this from a tactical point of view. Who is more diverted?
The United States has a large, traditional fighting force consisting of over a hundred thousand men and women. The terrorists are a loosely linked multinational network, consisting of only a few thousand fighters. Keeping our armed forces stationed in Iraq is costing hundreds of billions of dollars, and it is draining many of the military's capabilities, including its supply of willing volunteer soldiers and its ability to be engaged elsewhere. The terrorists' decision-making is distributed, their costs are low (particularly when they are able to forage for weapons in the surrounding countryside), and small groups of them can come and go as they choose, in a matter of days. The military's comings and goings, conversely, constitute a complex and time-consuming logistical operation. Which side is likely to be more seriously diverted by war in Iraq? Which side can recover more swiftly from the distraction and, if it chooses, focus its attention elsewhere?
It is true that both sides entered the war by choice. To get the terrorists to engage us in Iraq, we had to make it a juicy enough target for them, but they still came in by choice. However, both sides do not have the option of leaving the war by choice. Although the United States could theoretically leave any time it wanted to, in reality the US is not free to go because its mission is so closely tied to the events on the ground in Iraq. The only way to leave Iraq before things have advanced to the point where there is a stable, palatable government is to concede at least partial failure and to go back home. The terrorists, however, are free to go operate elsewhere. Even if they give up in Iraq and the US gets what it wanted, they are not failures at their mission as long as they contiue to act elsewhere. If it was worth diverting the terrorists into Iraq, then they must have other ripe targets once they are undiverted. For the terrorists, exit could be an opportunity to act elsewhere, so it could come swiftly with a minimum of bad publicity for their cause, while for the United States it could be a slow, embarrassing retreat.
The final point to be made on the war in Iraq is this: the battle is not merely between the two fighting forces that entered Iraq, the United States (and its allies) on one side and the terrorists on the other. Both sides are receiving support from among the people of Iraq and others outside Iraq, recruiting and training men to serve on their side. The US is trying to train Iraqi security forces, while the terrorists are joined in their fight by various sets of insurgents. Some of the men on either side come from preexisting fighting forces, including former soldiers who now work for the security forces and former Baathists who are now insurgents. The fighting inspired others to join up, as the violence and destruction around them provided people with motivation either to join the security forces or to side with the insurgency, depending on one's interpretation of where the damage is coming from. Despite these apparent symmetries, the two sides are in very different positions. The allied forces have a much harder job. They must gather a much larger fighting force, and they must train them directly to do a very difficult job. Corruption, bad discipline, and thuggery could all seriously undermine the value of the security forces, as could the presence of traitors. The insurgents do not need to be nearly as numerous, and their decentralized nature seems to make them more resilient to the presence of "bad apples" in their midst. A diversion does not accomplish much if the "diverted" people can multiply while they are diverted, and in Iraq the terrorists seem to be in a much better recruiting position than the Americans.
A brief disclaimer "below the fold" (i.e. in the comments)