"Let me say this as plainly as I can," he said. "By August 31, 2010, our combat mission in Iraq will end."
Yet in the same speech before Marines and military leadership, he said the vast majority of those involved in the pullout will not leave this year. Mr. Obama also said tens of thousands of U.S. personnel will remain behind to train and advise Iraqis.
"We have forged hard-earned progress, we are leaving Iraq to its people, and we have begun the work of ending this war," he said.
Mr. Obama was moving to fulfill in large measure the defining promise of his presidential campaign - to end combat operations within 16 months of taking office. He's doing it in 19 months instead.
The president said the U.S. cannot "let the pursuit of the perfect stand in the way of achievable goals." That was a declaration not of mission accomplished, but of mission accomplished as best as America could - this in the face of Mr. Obama's growing commitment to the conflict in Afghanistan, the other war he inherited.
"The most important decisions that have to be made about Iraq's future must now be made by Iraqis," the president said at the sprawling Camp Lejeune, N.C., base, which is about to deploy thousands of troops to Afghanistan.
Senior Obama administration officials had said earlier that of the roughly 100,000 U.S. combat troops to be pulled out of Iraq over the next 18 months, most will remain in the war zone through at least the end of this year to ensure national elections there go smoothly.
The pace of withdrawal means that although the pullout will start soon, it will be backloaded, with most troops returning in the last few months of the time frame.
And even after the drawdown, a sizable U.S. force of 35,000 to 50,000 U.S. troops will stay in Iraq longer under a new mission of training, civilian protection and counterterrorism.
"The Pentagon will never leave troops in a war zone without giving them the ability to defend themselves," CBS News national security correspondent David Martin told Political Hotsheet. "In other words, those stay-behind troops, although they will be designated as 'training and assistance' brigades, will be fully combat-capable."
The residual troop force will have three missions, reports CBS News chief White House correspondent chip Reid: To train and advise Iraqi forces; to protect the civilian population; and to conduct operations against suspected terrorists.
In any case, all U.S. troops must be out of Iraq by Dec. 31, 2011. That's the deadline set under an agreement the two countries sealed near the end of Bush's presidency. Mr. Obama has no plans to extend that date or pursue any permanent troop presence in Iraq.
But Defense Secretary Robert Gates indicated Friday that the U.S. will be prepared to stay beyond the 2011 deadline, provided the Iraqis asked them.
More than 4,250 Americans have been killed in Iraq, a costly, unpopular enterprise at home that Mr. Obama criticized when support for the invasion was strong and few other politicians dared stand against it.
Mr. Obama's strong opposition to the war helped propel him to the White House. During the campaign, he promised to withdraw combat troops within 16 months and to move quickly, pulling out at least one brigade a month, Reid reports.
But after extensive consultations with the Pentagon, including Defense Secretary Robert Gates, a Bush administration hold-over, the president decided to give commanders more flexibility, Reid reports.
"He said all along, even in the campaign that he would listen to the commanders in the field," Gates said. "And I think he did that."
Despite the extra months he's taking to achieve a withdrawal at the advice of military commanders, it is a hastier exit than envisaged under his predecessor, George W. Bush, whom Mr. Obama called Friday before giving his speech.
"America can no longer afford to see Iraq in isolation from other priorities," Mr. Obama told the crowd. "We face the challenge of refocusing on Afghanistan and Pakistan; of relieving the burden on our military; and of rebuilding our struggling economy and these are challenges that we will meet."
He applauded the armed forces for successes in Iraq, where U.S. deaths and violence in many parts of the country are significantly down.
Yet he acknowledged violence will remain "a part of life" and daunting problems include political instability, displaced citizens, lack of support for Iraq's government in the neighborhoods and the stress of declining oil revenues.
But, the president said the U.S. cannot continue to try to solve all Iraq's problems.
"We cannot rid Iraq of all who oppose America or sympathize with our adversaries," he said. "We cannot police Iraq's streets until they are completely safe, nor stay until Iraq's union is perfected. We cannot sustain indefinitely a commitment that has put a strain on our military, and will cost the American people nearly a trillion dollars."
The American public also views the progress in Iraq more favorably than a year ago. According to a CBS News/New York Times poll released this week, 63 percent of Americans say the situation in Iraq is going very or somewhat well.
And Sen. John McCain, Mr. Obama's Republican presidential rival, endorsed Mr. Obama's plan in a striking closing of ranks between opponents who argued forcefully over Iraq's course in the campaign.
House Speaker Nancy Pelosi, D-Calif., called the announcement good news because it means an end to the war. At the same time, she said the troops left behind must have "clearly defined" missions "so that the number of troops needed to perform them is as small as possible."
Defense Secretary Robert Gates and Adm. Michael Mullen, chairman of the Joint Chiefs of Staff, told lawmakers that ground commanders in Iraq believe the plan poses only a moderate risk to security, said McCain, the top Republican on the Senate Armed Services Committee.
House Republican Leader John Boehner of Ohio also approved. "I believe he has outlined a responsible approach that retains maximum flexibility to reconsider troop levels and to respond to changes in the security environment should circumstances on the ground warrant," he said.
Mr. Obama responded to his congressional critics in an interview to be broadcast Friday on PBS' "The Newshour," saying that maybe they weren't paying attention to his comments during two years of campaigning for the presidency.
"Everything that I said I would do during the campaign I am now doing," Mr. Obama said. "Obviously because of consultation with commanders on the ground, something I also said we would do, there are some modifications to the plan. But this is basically the thrust that I have been talking about for several years and I think it is a responsible solution."
Mr. Obama wants to keep a strong security presence in Iraq through a series of elections in 2009, capped by national elections tentatively set for December. That important, final election date could slip into 2010, which is perhaps why Mr. Obama's timetable for withdrawing combat troops has slipped by a few months, too.
U.S. To Intensify Its Diplomatic Efforts
Mr. Obama said the troop drawdown would be coupled with increased diplomatic efforts that will amount to a "new era of American leadership and engagement in the Middle East."
Mr. Obama said the U.S. would "pursue principled and sustained engagement" with all nations in the region, including Syria and Iran, in order to stabilize Iraq.
The troop drawdown is also a necessity, said the president, both for the future of Iraq and to allow the U.S. to refocus its attention more firmly on Afghanistan.
"America can no longer afford to see Iraq in isolation from other priorities: we face the challenge of refocusing on Afghanistan and Pakistan; of relieving the burden on our military; and of rebuilding our struggling economy and these are challenges that we will meet," he said.
"Every nation and every group must know, whether you wish America good or ill, that the end of the war in Iraq will enable a new era of American leadership and engagement in the Middle East," he said. "This does not lessen our commitment. We are going to be enhancing that commitment to bring about a better day in that region, and that era has just begun."