BAGHDAD — The United States changed commanders in Iraq today, beginning the final phase of American military involvement in the country despite political uncertainty and persistent violence.

The transfer of authority came a day after 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.

Newly promoted Army Gen. Lloyd Austin maintained a somber tone as he took the reins of the 50,000 American troops who remain in Iraq, with a deadline for a full withdrawal by the end of next year.

He noted "hostile enemies" continue to threaten Iraq and pledged that "our national commitment to Iraq will not change."

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.