Critics like Mr. Nagl argue that the anniversary should serve as a reminder about what he sees as the mistake of starting the war. “It would be a shame if we did not pause and think hard about this as a nation,” he said. “We paid an enormous price as a nation. The Iraqis have paid a huge price. The region is destabilized.”

Some war supporters disagreed. “President Bush made the right decision on removing the Iraqi regime from power,” said Douglas J. Feith, a former under secretary of defense. Where America went wrong, he said, was “when we transformed ourselves from liberators to occupiers.” Like other war supporters, he expressed concern that Mr. Obama might lose the peace. “The full withdrawal of U.S. forces is risky,” he said.

Meghan O’Sullivan, a former deputy national security adviser to Mr. Bush, said that some lessons could be drawn, but that it was too soon for final judgments. “Many issues that will be key to answering the question of was it worth it still hang in the balance,” she said.

Public attitudes toward the war have hardened 10 years later. Fifty-four percent of Americans interviewed by CBS News in a poll released Tuesday said the United States should have stayed out of Iraq, while 38 percent said it did the right thing. Fifty percent said the United States did not succeed in achieving its objectives, while 41 percent said it did.

The White House found itself in the awkward position of standing by Mr. Obama’s opposition to the war but offering an optimistic prognosis for Iraq — and even giving a grudging nod to Mr. Bush for removing a dictator from power.

“Ridding the world of Saddam Hussein was a welcome development for the world and for Iraq,” Jay Carney, the White House press secretary, said at his daily briefing. “The president believes that Iraq has the potential for a better future today because of the remarkable sacrifice and service of American men and women in uniform as well as civilian American men and women who served in Iraq.”

Asked if that better future owed in part to the decision to invade, Mr. Carney said Mr. Hussein was removed by the military sent by Mr. Bush.

“And to the extent that credit is due, credit is due to him for that,” Mr. Carney said. “That does not change, I think, assessments made by this president as a candidate or by many others on this day, 10 years after, about the judgments made to go to war in Iraq.”