U.S. soldiers attend the United States Forces-Iraq change of command ceremony in Baghdad on Wednesday. (Hadi Mizban/Associated Press)

The United States changed commanders in Iraq Wednesday, marking the start of the final phase of its military involvement in the country despite political uncertainty and persistent violence.

U.S. Army Gen. Lloyd Austin maintained a sombre tone as he took the reins of the nearly 50,000 American troops who will remain in Iraq in a non-combat role. The deadline for their withdrawal is the end of 2011.

Austin noted that "hostile enemies" continue to threaten Iraq and pledged no change in the American commitment there.

"Although challenges remain, we will face these challenges together," Austin said during the ceremony, which was held at the main U.S. military headquarters on the southwestern outskirts of Baghdad.

Austin, who most recently served in Iraq as commander of troop operations from 2008-09, replaces Gen. Ray Odierno, who is heading to Virginia to take over the Joint Forces Command after a total of about five years in Iraq.

War 'clouded' by its start: Gates

The transfer of authority came less than a day after U.S. President Barack Obama announced the shift from combat operations to preparing Iraqi forces to assume responsibility for their own security. Obama made clear in Tuesday's speech that this was no victory celebration.

From left, U.S. Vice President Joe Biden, U.S. Secretary of Defence Robert Gates and Chairman of the Joint Chiefs Admiral Mike Mullen stand during the playing of their national anthem at Wednesday's transfer ceremony. (Jim Watson/Associated Press)Vice-President Joe Biden, Defence Secretary Robert Gates and Joint Chiefs Chairman Admiral Mike Mullen presided over the ceremony.

Gates, visiting American troops in the Iraqi city of Ramadi Wednesday, said history will judge whether the fight was worth it for the United States.

"The problem with this war, I think, for many Americans, is that the premise on which we justified going to war turned out not to be valid," he said. "Even if the outcome is a good one from the standpoint of the United States, it'll always be clouded by how it began."

In 2003, then President George W. Bush ordered the invasion of Iraq — with the approval of Congress — based on claims Saddam was holding weapons of mass destruction.

The claims turned out to have been based on faulty intelligence; the weapons were never found.

'Time to turn the page'

In declaring the end to combat Tuesday, Obama also praised American forces for their work. He acknowledged the war's ambiguous nature, in which American forces quickly ousted Saddam but were never able to fully control the Sunni Muslim insurgency against the Shiite-dominated establishment that even now threatens to re-ignite.

U.S. President Barack Obama spoke from the Oval Office at the White House about the end of the U.S. combat role in Iraq in Washington, D.C., on Tuesday night. (APTN/Associated Press)

Still, he said the time had come to close this divisive chapter in U.S. history.

"We have met our responsibility," Obama said. "Now it is time to turn the page."

Avoiding any hint of claiming victory in a war he once called a major mistake, the president recognized the sacrifices of America's military. More than 4,400 American troops and an estimated 100,000 Iraqis were killed and it cost billions of dollars.

Iraqi Prime Minister Nouri al-Maliki, a Shiite, said Tuesday the end of combat operations was a return to sovereignty and independence for the hobbled country, and tried to reassure his people that their own security forces will be able to defend them.

Iraqi forces on Wednesday appeared to be on heightened alert, spread out at checkpoints across the city intended to reassure the populace and ward off insurgent attacks.