3 Most likely scenarios that could happen in Iraq
These are the 3 possible scenarios that I think could happen for Iraq given the parameters mentioned. Although this is only an educated guess in my part, the phenomenon of Mahdism has to be factored in when projecting any future dealing with the Arab/Islamic world. This knowledge is what most commentators lack when dealing with projecting any future for this part of the world.
The best one we can hope for is that somehow democracy works in Iraq and miraculuosly catches on throughout the Middle East despite the disunity of the West. Or it may be that the entire West comes together to see this through. This gives birth to a new era in the region, one of tolerance and openness, almost guaranteeing a future of peace from the region. This however is the most unlikely scenario an is definitely a long shot, but still has a slim chance of happening. Definitely the best case scenario, let's all hope that this is the one that happens, although I tend to be more realistic. But even with this occurring there is no guarantee that Mahdism will not pose a problem in the future, albeit less so.
The opposite could occur giving us a worst case scenario. With few willing to stand with the US, the Iraqi democracy collapses within a year or two after a US withdrawal resulting in civil war. All hope for democracy within the region is lost, with little hope of a better tomorrow people turn to the only other thing there is--- fundamentalist Islam. By 2011 the entire muslim world is under the grip of fundamentalist radicals, its only a matter of time before the rest of the world will see what this means.
The third case scenario, the one I believe the most likely to happen.
A democracy is introduced in Iraq and is stable for around a decade or so. The moderates will gain some influence and they will slowly make strides. After this time the fundamentalist radicals slowly gain ground again because a disunited West did not help the US make a sweeping change to the region. The region is split between the moderates and the fundamentalist, but with the more radical fundametals willing to take extreme action the moderates keep getting pushed back. With the region almost equally divided the coming of the Mahdi results in a regional war, in which the radicals are most likely to win because of their more fanatical nature, causing the whole region to now come under fundamentalist radical rule. Once the regional war is over the newly formed Islamic empire prepares a massive military buildup for a war unlike anyone has ever seen, a war they believe they will win. Flashback to the present; because of Bush's decision and the US and (staunch but few) allied effort in Iraq at the moment, this has bought us time, some 10 to 20 years or more before the this actually happens, which isn't much, but better than nothing.