Washington invaded Iraq in 2003 to topple Saddam Hussein. Violence has dropped since sectarian slaughter in 2006-2007, but Sunni insurgents and Shi'ite militia still mount attacks.

Here is a timeline on U.S. forces in Iraq since 2003:

March 20, 2003 - U.S.-led forces invade Iraq from Kuwait to oust Saddam Hussein.

-- About 125,000 U.S. and British soldiers and Marines are in Iraq. By the end of April, U.S. says it will add 100,000 more soldiers to the U.S.-led invasion force.

April 9 - U.S. troops take Baghdad, Saddam goes into hiding.

May 1 - President George W. Bush declares hostilities over.

-- Between March 20 and May 1, 138 U.S. troops are killed.

December 13 - U.S. troops capture Saddam near Tikrit.

February 22, 2006 - Bombing of Shi'ite shrine in Samarra sparks widespread sectarian slaughter, raising fears of civil war.

February 14, 2007 - Prime Minister Nuri al-Maliki launches U.S.-backed crackdown in Baghdad aimed at pulling Iraq back from brink of civil war.

-- Five U.S. combat brigades plus supporting troops, about 30,000 soldiers, are sent to Iraq between February and mid-June 2007. Besides reducing violence, Washington wanted to create a "breathing space" for Iraqi leaders to make progress on laws seen as critical to fostering national reconciliation.

June 15 - U.S. military completes its troop build-up, or "surge," to around 170,000 soldiers.

-- From April to June 2007, 331 U.S. soldiers are killed, the deadliest quarter of the war for the U.S. military.

August 29 - Anti-U.S. Shi'ite cleric Moqtada al-Sadr orders his Mehdi Army militia to cease fire.

September 10 - U.S. commander in Iraq, General David Petraeus, recommends cutting troops by more than 20,000 by mid-2008.

July 22, 2008 - The U.S. military says the last of five extra combat brigades sent to Iraq in 2007 have withdrawn, leaving just under 147,000 U.S. troops in Iraq.

November 17, 2008 - Iraq and the United States sign an accord requiring Washington to withdraw its forces by the end of 2011. The pact gives the government authority over the U.S. mission for the first time, replacing a U.N. Security Council mandate. Parliament approves pact after negotiations 10 days later.

February 27 - New U.S. President Barack Obama announces plan to end U.S. combat operations in Iraq by August 31, 2010, but says he will leave up to 50,000 troops to train Iraqi forces.

June 4, 2010 - U.S. military says there are 88,000 troops in Iraq.

June 30 - All U.S. combat units withdraw from Iraq's urban centers and redeploy to bases outside.

August 31 - Troops levels in Iraq are reduced to 49,700 after a major draw down is completed.

April 26, 2011 - U.S. Congressional Budget Office says the cost of Iraq operations totals around $752 billion since 2003.

May 3 - A "small, residual" U.S. force should remain in Iraq beyond the end of 2011, after the scheduled withdrawal, U.S. House Speaker John Boehner says.

June 6 - In the biggest single loss of life since 2009, six U.S. soldiers are killed in a rocket attack on a Baghdad base.

October 4 - Maliki wins support from political blocs on keeping U.S. troops as trainers, but they reject any deal that would grant U.S. troops immunity as Washington had requested.

October 21 - Obama says he will pull all U.S. troops out of Iraq at the end of 2011, dashing U.S. hopes of leaving a few thousand troops to buttress a still shaky Iraq, after the failure of negotiations on keeping some there as trainers.

November 23 - About 700 U.S. trainers, mainly civilians, will help Iraqi security forces when American troops leave, Iraqi and U.S. officials say.

December 2 - The U.S. military vacates Victory Base Complex, its vast main base near Baghdad airport that was once the hub of the American war operation.

December 12 - NATO says it will end its seven-year troop training mission in Iraq at the end of December, to coincide with the withdrawal of U.S. troops.