Today, he promised to end the war and shift resources to fighting al Qaeda and the Taliban in Afghanistan.
Obama, embroiled in sharp debate with Republican White House rival John McCain over Iraq, said the lengthy commitment of combat troops there diminished US security and standing in the world.
"By any measure, our single-minded and open-ended focus on Iraq is not a sound strategy for keeping America safe," Obama said in a speech designed to lay out his views on the war ahead of his planned trip to Afghanistan and Iraq soon.
"As president, I will make the fight against al Qaeda and the Taliban the top priority that it should be," Obama said. "This is a war that we have to win."
The future of Iraq promises to be a central issue in the November election battle for the White House between McCain, a staunch advocate of the war, and Obama, an early opponent of the US invasion.
Obama criticised McCain, an Arizona senator, and US President George W. Bush for making Iraq the center of the battle against terrorism and said he would pursue a new national security strategy that rebuilt foreign alliances and regained global goodwill destroyed by the war.
"I am running for president of the United States to lead this country in a new direction - to seize this moment's promise," the first-term senator from Illinois said.
Obama also promised a renewed effort to seize nuclear materials from terrorists and "rogue" nations, and said he would be willing to use "all elements" of US power to pressure Iran on its nuclear program.
McCain has suggested Obama is wavering on his pledge to withdraw US combat troops from Iraq within 16 months, pointing to Obama's recent comment that he might "refine" his policies on Iraq based on consultations with military commanders.
But Obama emphasised that while he is open to tactical adjustments, the 16-month timetable remains his goal. He said their disagreement on Iraq was indicative of a different approach to diplomacy and national security.
"Senator McCain wants to talk of our tactics in Iraq; I want to focus on a new strategy for Iraq and the wider world," he said.
The renewed debate on Iraq came as bombers killed around 40 people and wounded scores in several attacks in northern Iraq, days after the government vowed to expand a crackdown against militants in a region where al Qaeda retains influence.
McCain criticised Obama for giving a speech on Iraq before traveling there. Obama's visit to Iraq, where he has only been once, in 2006, and Afghanistan follows repeated criticism from McCain that he should visit the area and talk to commanders.
"Senator Obama is departing soon on a trip abroad that will include a fact-finding mission to Iraq and Afghanistan," McCain said in written excerpts of comments he will deliver later on Tuesday in New Mexico.
"And I note that he is speaking today about his plans for Iraq and Afghanistan before he has even left," McCain said. "In my experience, fact-finding missions usually work best the other way around: first you assess the facts on the ground, then you present a new strategy."
A Washington Post/ABC News poll released on Monday found Americans evenly divided on the candidates' positions on Iraq with 47 per cent of those polled saying they trust McCain more to handle the war, and 45 per cent having more faith in Obama.
The poll showed the public split on whether Obama could serve effectively as commander in chief with 48 per cent saying he would be an effective leader of the military and 48 per cent saying he would not.
Obama has proposed adding two US combat brigades, about 9,000 troops, to the 36,000 troops already in Afghanistan and said this would be made possible by a drawdown of troops from Iraq.
He also called for changes to US policy toward Pakistan, saying Bush had offered a "blank check" to Pakistani President Pervez Musharraf in the form of US military aid.
Many Taliban and al Qaeda militants are believed to be hiding in Pakistan along the rugged border area near Afghanistan. Obama called for tripling nonmilitary aid to Pakistan and said he would co-sponsor a bill to do so.
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