But 3 1/2 years later, with no weapons found, still no end in sight and the war a liability for nearly all Republicans on the ballot Nov. 7, the justification has become far broader and now includes the expansive "struggle between good and evil."
When no weapons of mass destruction were found in Iraq, Bush shifted his war justification to one of liberating Iraqis from a brutal ruler.
After Saddam's capture in December 2003, the rationale became helping to spread democracy through the Middle East. Then it was confronting terrorists in Iraq "so we do not have to face them here at home," and "making America safer," themes Bush pounds today.
Bush at first sought to explain increasing insurgent and sectarian violence as a lead-up to Iraqi elections. But elections came and went, and a democratically elected government took over, and the sectarian violence increased.
Bush has insisted U.S. soldiers will stand down as Iraqis stand up. He has likened the war to the 20th century struggles against fascism, Nazism and communism. He has called Iraq the "central front" in a global fight against radical jihadists.
Having jettisoned most of the earlier, upbeat claims of progress, Bush these days emphasizes consequences of setting even a limited withdrawal timetable: abandonment of the Iraqi people, destabilizing the Middle East and emboldening terrorists around the world.
The more ominous and determined his words, the more skeptical the American public appears, polls show, both on the war itself and over whether it is part of the larger fight against terrorism, as the administration insists.
Bush's approval rating, reflected by AP-Ipsos polls, has slid from the mid 60s at the outset of the U.S.-led invasion in March 2003 to the low 30s now.
Democrats say Iraq has become a distraction from the war against terrorism — not a central front.
More than 2,750 members of the U.S. military have died since the beginning of the war, most of them since Bush's May 2003 "mission accomplished" aircraft carrier speech. Hundreds of thousands of Iraqis have died.
Not all of Bush's rhetorical flourishes have had the intended consequences.
When the history of Iraq is finally written, the recent surge in sectarian violence is "going to be a comma," Bush said in several recent appearances. Critics immediately complained that the remark appeared unsympathetic and dismissive of U.S. and Iraqi casualties.
For a while last summer, Bush depicted the war as one against "Islamic fascism," borrowing a phrase from conservative commentators. The strategy backfired, further fanning anti-American sentiment across the Muslim world.