The United States is marking the end of its military mission in Iraq ahead of a deadline to finish removing its troops by December 31. Here is a look at key events since the 2003 invasion.

March 20, 2003: U.S. forces invade Iraq.

April 9, 2003: U.S. forces pour into Baghdad.

May 1, 2003: President George W. Bush says major combat operations are over in Iraq.

December 13, 2003: Saddam Hussein is captured near his hometown of Tikrit, north of Baghdad.

December 30, 2006: Saddam is executed.

January 10, 2007: President Bush announces a surge of more than 21,000 troops to combat growing violence.

July 22, 2008: Troop surge ends, leaving about 147,000 U.S. troops in Iraq.

November 27, 2008: Iraqi parliament approves U.S.-Iraq security agreement which sets a date of December 31, 2011 for all U.S. forces to leave Iraq.

February 27, 2009: President Barack Obama says all but 50,000 troops will withdraw by August 31, 2010.

June 30, 2009: U.S. combat troops leave Iraqi cities, handing security to Iraqi forces.

August 31, 2010: President Obama announces end of U.S. combat mission in Iraq. About 50,000 troops remain to train and advise Iraqi forces.

December 21, 2010: After nine months of delays, Iraq approves a new government.

August 02, 2011: Iraqi political leaders agree to negotiate a possible deal that would allow some U.S. troops to remain after December on a training mission.

August 15 2011: Bombings and attacks in 17 Iraqi cities leave more than 60 people dead, raising concerns about security after the U.S. pullout.

September 1, 2011: The U.S. military says no American service members died in Iraq in August, the first month with no U.S. military fatalities there.

October 21, 2011: President Obama confirms all U.S. troops with be withdrawn by the end of the year.

November 29, 2011: U.S. Vice President Joe Biden tells Iraqi leaders the two countries are embarking on a “new path” as the U.S. nears completion of its withdrawal.

December 14, 2011: Days after meeting with Iraqi Prime Minister Nouri al-Maliki, President Obama says U.S. troops are leaving behind a sovereign, stable and self-reliant Iraq.

December 15, 2011: U.S. Secretary of Defense Leon Panetta travels to Baghdad for a ceremony marking the end of the U.S. mission in Iraq.