Timeline: Invasion, surge and withdrawal in Iraq

(Reuters) - The United States is on track to draw down its forces in Iraq to 50,000 by August 31 when U.S. combat operations are due to finish as part of a pledge by President Barack Obama to end the war.

The 50,000 remaining will train Iraqi armed forces and police units ahead of a full withdrawal by the end of 2011.

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.

March 23, 2003 - On the third day of the war, 28 U.S. soldiers are killed, mostly in fierce fighting in southern Iraq.

April 9 - U.S. troops take Baghdad, Saddam disappears.

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.

November 8, 2004 - U.S. troops begin a second assault in Falluja - a Sunni Muslim city 30 miles west of Baghdad - intended to deprive insurgents of a safe haven from which American officials said rebels had coordinated a spree of killings, bombings and kidnappings. More than 71 U.S. Marines and more than 1,000 guerrillas were killed in the 10-day attack.

January 26, 2005 - U.S. Marine transport helicopter ferrying troops comes down in western desert, killing 31 U.S. troops in the deadliest single incident for Americans in Iraq. Six more are killed in insurgent attacks.

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

February 14, 2007 - 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, or some 30,000 soldiers, are sent to Iraq between February and mid-June 2007. Besides reducing violence, Washington wanted to create "breathing space" for Iraqi leaders to make progress on laws seen as critical to fostering national reconciliation.

June 15 - U.S. military says it has completed its troop buildup, or "surge," to 160,000 soldiers.

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

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.

January 1, 2009 - U.S.-Iraq security pact comes into force, placing 140,000 U.S. troops in Iraq under Iraqi authority.

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

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

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

August 7 - The 4th Stryker Brigade, 2nd Infantry Division, the last brigade mainly focused on combat, hands over to Iraqi forces.

August 18 - U.S. troop strength in Iraq is 56,000, a senior Obama administration official says.

August 31 - U.S. to cut its troop levels to 50,000.