WASHINGTON -- Two years after President Bush decided to go to war against Iraq, the decision itself -- more than the war or the insurgency that followed -- remains the defining event of his presidency.
Bush's skepticism of international alliances and institutions -- now the most striking facet of his foreign policy --was hardly visible before that decision. The earlier war against the Taliban regime in Afghanistan had been a model of painstaking international cooperation, with hostilities delayed until the United States could coordinate with a loose group of rebels known as the Northern Alliance.
Moreover, Bush's firmness of resolve -- the unwillingness to yield to critics that some people admire as courage and others deride as stubbornness -- was also less in evidence before the Iraq decision.
Over the first two years of his presidency, Bush strived to make good on his campaign promises to seek bipartisan agreement. He compromised with Democrats on the No Child Left Behind education law and welcomed leaders of the opposing party as partners in pursuing the war on terrorism.
The Iraq war frayed many alliances at home and overseas. But rather than try to rebuild them along the same lines as before, Bush declared that his first obligation was to do what he thought was best and worry about pleasing others later.
''I'm the kind of fellow who does what I think is right, and will continue to do what I think is right," Bush said at a news conference in Ottawa after his reelection last year. ''I'll consult with our friends and neighbors, but if I think it's right to remove Saddam Hussein for the security of the United States, that's the course of action I'll take."
No one can fully explain Bush's change in attitude, just like no one can exactly pinpoint why he felt the need to go to war when he did.
Many explanations were offered by the president, from thwarting weapons of mass destruction to disrupting an alleged alliance between Iraq and Al Qaeda, to saving the Iraqi people from tyranny.
Bush's supporters supplied some others -- from the ''reverse domino theory" of turning Iraq into a democracy in hopes that democratic fervor would spill over into neighboring countries to the idea that a show of force in Iraq would frighten other Mideast dictators into cooperating with the United States.
Bush's detractors have supplied some possible reasons of their own -- from a need to secure a steady source of Mideast oil to a desire to finish his father's fight with Hussein in a Texas-style face-off.
But whatever the reasons for the decision, Bush emerged from the process a changed leader. His final news conference before the war was an odd event, with an almost sullen Bush calling on reporters from a prearranged list and rarely deviating from a few talking points, such as demanding proof that Iraqi weapons had been destroyed and repeating his determination to ''disarm" Hussein.
Having failed to obtain United Nations approval for the war, he pushed the start button anyway, and a different Bush was on display almost immediately.
The president, who responded to the victory in Afghanistan with statesmanlike reserve, donned a flight suit to declare an end to ''major combat operations" in Iraq.
When victory proved more elusive, and some reasons for the war proved untrue, he stiffened his spine, insisted that he was not going to turn back, and resisted inviting nations that opposed the war to participate in the rebuilding of Iraq.
Bush behaved as if he were adhering to a long-planned strategy, even when the lack of preparedness on the ground indicated otherwise.
More likely, the pressure to justify his actions led to the creation of the plan, redefining his goal from eliminating weapons of mass destruction to promoting freedom.
On the campaign trail last year, he repeatedly proclaimed the need for unilateral actions to combat terrorist threats and promote democracy.
In hindsight, hints of this Bush were visible earlier in his presidency, in acts such as backing out of the Kyoto global warming accords and refusing to sign on to a world criminal court. But the decision to go to war in Iraq marked a clear turning point in his behavior, if not his thinking.
Supporters believe the Iraq decision alerted Bush to the limits of international consensus-building, how the process itself can impede bold action.
Critics see another reality -- that the nation's most important foreign-policy doctrines may have been forged out of a need to justify and defend a mistake.
Peter S. Canellos is the Globe's Washington bureau chief. National Perspective is his weekly analysis of events in the capital and beyond.