"Let me be clear: There is no military solution in Iraq and there never was," Obama was expected to say in a speech Wednesday at Ashford University.
"The best way to protect our security and to pressure Iraq's leaders to resolve their civil war is to immediately begin to remove our combat troops. Not in six months or one year - now," the Illinois senator was to say.
Obama's ardent opposition to the war has been a central theme of his campaign for the Democratic presidential nomination, and he has used it to distinguish himself from leading rival Sen. Hillary Rodham Clinton, a New York Democrat. She voted to authorize the use of force in Iraq; Obama was not yet a senator.
Obama was trying to further sharpen that distinction Wednesday, spelling out his views on what the U.S. should do next.
He introduced legislation in January calling for withdrawal to start on May 1 and for all combat brigades to be pulled out by March 31, 2008.
Anti-war Democrats and some Republicans want to bring all combat troops home in a matter of months.
Obama's speech comes after Gen. David Petraeus, the top U.S. commander in Iraq, and U.S. Ambassador Ryan Crocker on Monday updated Congress on the situation in the war zone during two days of testimony.
Petraeus recommended that a 2,000-member Marine unit come home this month and not be replaced. That would be followed in mid-December by the departure of an Army brigade of 3,500 to 4,000 soldiers. An additional four combat brigades would be withdrawn by July 2008.
Obama said the U.S. and the Iraqi government should discuss how to go about withdrawing troops.
"We must get out strategically and carefully, removing troops from secure areas first and keeping troops in more volatile areas until later," Obama said in prepared remarks. Key excerpts were obtained by The Associated Press.
Although he stopped short of calling for an immediate pullout of all troops, Obama said there should be a clear and certain timetable.
"But our drawdown should proceed at a steady pace of one or two brigades each month," he said. "If we start now, all of our combat brigades should be out of Iraq by the end of next year."
By arguing that only combat brigades should be withdrawn - there are 20 in Iraq, including five sent in January by President George W. Bush - Obama appeared to suggest that other U.S. troops could remain.
Underscoring the importance he was putting on the speech, Obama was being introduced by Zbigniew Brzezinski, who was national security adviser to President Jimmy Carter from 1977 to 1981. Brzezinski has endorsed Obama's bid, and Wednesday's appearance would be his first on the candidate's behalf.
Obama rejected Petraeus' recommendation to maintain current troop levels through next summer to ensure security gains are maintained.
"The president would have us believe there are two choices: keep all of our troops in Iraq or abandon these Iraqis," Obama said. "I reject this choice."
Instead, he argued for creating an international working group of countries in the region and in Asia and Europe that would work to stabilize Iraq.
Obama called for increasing U.S. aid to US$2 billion (EUR1.5 billion) to support such an effort, to expand access to social services for Iraqi refugees in neighboring countries and to assure that Iraqis displaced inside their own country can find safe haven. He also wants a commission to monitor and hold accountable perpetrators of war crimes in Iraq.
Obama's speech marked the beginning of a two-day trip to Iowa. Most polls show he, Clinton and former Sen. John Edwards in a close race in the state, where precinct caucuses launch the presidential nominating process.