There appears to be two ideas on how to deal with the aftermath of 911. One that it is a law enforcement issue dealing with a band of criminals. The other that it is a war against a virtual state headed by OBL.
Law enforcement deals with piecing together the evidence and bringing the criminals to justice. Warfare deals with eliminating the immediate threat and deterring future threats. The main problem with the law enforcement idea is that Western justice has very little deterrence value and notions of justice prevent you from attacking targets that are not directly related to the crime.
The advantage of the warfare model is that action can be taken that is possibly illegal yet has great strategic worth. Thus an unrelated country could be invaded if it had strategic value in the war. War has little to do with law but should have everything to do with an effective strategy.
Bush no doubt had a strategy planned long before 911 as 911 was simply the largest attack in a long string of attacks. There is no reason to believe that the attacks will abate therefore war is inevitable. Saddam can consider himself to be collateral damage in the GWOT. Could not have happened to a nicer bloke.