he wait for the UN inspectors?
Also, it isn't just about the WMDs. Bush received two reports that said pretty much the same thing about an invasion:
At the time, the White House and the Pentagon were saying that U.S. troops would be greeted as liberators, democracy would be quickly established and Iraq would become a model for the Middle East. Initial post-invasion plans called for U.S. troop withdrawals to begin in summer 2003.
The classified reports, however, predicted that establishing a stable democratic government would be a long challenge because Iraq's political culture did "not foster liberalism or democracy" and there was "no concept of loyal opposition and no history of alternation of power."
They also said that competing Sunni, Shiite and Kurd factions would "encourage terrorist groups to take advantage of a volatile security environment to launch attacks within Iraq." Because of the divided Iraqi society, there was "a significant chance that domestic groups would engage in violent conflict with each other unless an occupying force prevented them from doing so."
While predicting that terrorist threats heightened by the invasion would probably decline within five years, the assessments said that lines between al-Qaeda and other terrorist groups around the world "could become blurred." U.S. occupation of Iraq "probably would boost proponents of political Islam" throughout the Muslim world and "funds for terrorist groups probably would increase as a result of Muslim outrage over U.S. actions."
http://www.washingtonpost.com/wp-dyn/content/article/2007/05/25/AR2007052501380.html
The Bush/Cheney administration had read these two reports (which were the general concensus of all the gov't agencies involved) which said that any involvement would be lengthy, while telling Americans it would take months. I think one of the reports even mentions a time frame of between 5-10 years.
Bush Sr. had these same reports/assessments which is why he didn't go into Iraq and take out Saddam. He took a lot public flak for that decision, but I do respect that he made the right decision in the face of a disapproving public. Something Jr. didn't do if he went in because of the "momentum".