Steven Den Beste has a typically long, but well-reasoned piece on why Iraq is a logical extension to the war on terror.
What must be understood is that we are not just at war with al-Qaeda. Just removing al-Qaeda is only a symptomatic treatment to a larger disease. That disease is the fundamental cultural failure that has enveloped much of the Arab world. We have both pragmatic and moral grounds to intercede in the region. The pragmatic ground is that if we do not, where al-Qaeda may disappear or die, others will take their place if the conditions that spawned them are not alleviated. The moral reason is equally compelling: the people of the Islamic world deserve better than the inhumanities of Wahhabism and shari’a.
This is similar to the "root causes" argument that was in fashion among the left after September 11. It does acknowledge that the failure of the Arab world is the concern of the United States, and we should do everything in our considerable power to bring freedom to the Arab world. However, we do not do this by dropping support of Israel or doing nothing in the face of terrorist aggression. We do this by being proactive – supporting democracy in the region by undermining the forces holding it back. We’re in a much larger war than the left (and some on the right) will admit. Either we act decisively now and bring the Arab world into the 21st Century, or we face decades of terror in which September 11 becomes the first of many atrocities. Given the choice, it is clear that our nation cannot afford the luxury of complacency, even if the actions that we must take are long and difficult.