GWB is often described by his supporters as a religious man. Many Americans take religion seriously. While Catholics are a minority in America, their doctrine on what constitutes a just war is well developed and not challenged by that of most Christian denominations.
The appropriate section of The Catechism of the Catholic Church
When is war justified? - From AmericanCatholic.com.
What has the Pope and the Church Said?
Statement on Iraq - United States Conference of Catholic Bishops Washington, D.C. November 13, 2002, which explains the just war theory and the burdens it places on countries and leaders who start wars.
Statement on Iraq - Most Reverend Wilton D. Gregory President United States Conference of Catholic Bishops - February 26, 2003
I could link to much more (such as this), but the Pope directly, clearly and frequently opposed GWB's rush to war and made his opposition plain. Why did so few American Catholics follow him? Some prominent Catholic neo-conservatives reinterpreted the Church's just war theory, trimming their moral sails to fit the political winds. Many Catholics supported their president rather than their Pope in a fit of patriotism, being unable to believe that a president would lead them into a war without some (hidden from them) justification.
Other Catholic Views
A Year After Iraq: Catholic Just War Doctrine
Bush vs. Benedict: Catholic neoconservatives grapple with their church’s Just War tradition.
I have not always agreed with the particulars and emphases of the Vatican, but rejecting, reshaping or ignoring the Church's teaching on just wars is a major slice of cafeteria Catholicism. It is no accident that Cindy Sheehan (and her son Casey) were both active in their parish.
National Guard Dad - August 20, 2005