About 39,000 troops are in Iraq. The U.S. had wanted to wanted to keep between 3,000 to 5,000 troops there past 2011 for help with training and to maintain security. But the current Status of Forces Agreement with Iraq dictates that the U.S. troops leave by year's end. CNN learned exclusively that the U.S. and Iraq had been unable to come to an agreement on key issues regarding legal immunity for U.S. troops remaining in Iraq, an impasse that effectively ended discussion of maintaining a significant American force presence beyond 2011.
CNN looks back at the events leading up to the war and its developments over the years.
February 5, 2003: U.S. Secretary of State Colin Powell makes the case to the United Nations that Iraqi President Saddam Hussein poses an imminent threat.
February 14, 2003: U.N. Chief Weapons Inspector Hans Blix reports to the U.N. Security Council that his team has found no weapons of mass destruction in Iraq.
March 17, 2003: President George W. Bush issues an ultimatum to Hussein and his family: Leave Iraq within 48 hours, or face military action.
March 19, 2003: At 10:15 p.m. EST, Bush announces that U.S. and coalition forces have begun military action against Iraq.
March 20, 2003: Hussein speaks on Iraqi TV, calling the coalition's attacks "shameful crimes against Iraq and humanity."
March 23, 2003: Members of the Army's 507th Maintenance Company are ambushed and captured outside Nasiriyah.
April 1, 2003: Pvt. Jessica Lynch is rescued from a hospital by U.S. forces.
April 9, 2003: Coalition forces take Baghdad, and a large statue of Hussein is toppled in Firdos Square. The White House declares "the regime is gone."April 13, 2003: Seven U.S. prisoners of war are rescued by U.S. troops.
May 1, 2003: Aboard the USS Abraham Lincoln, Bush declares "major combat operations" over, although some fighting continues.
May 22, 2003: The United Nations Security Council approves a resolution acknowledging the U.S. and Britain's right to occupy Iraq.
July 22, 2003: Hussein's sons, Uday and Qusay, are killed by U.S. forces.
December 13, 2003: Hussein is captured in a "spider hole" in Tikrit. This is not confirmed until December 14 by the U.S. Defense Dept.
March 8, 2004: The U.S.-backed Iraqi Governing Council members sign an interim constitution, laying the groundwork for future elections, a permanent constitution and eventually a return to self-rule.
June 1, 2004: The Iraqi Governing Council dissolves. An interim government will be in charge of the country after sovereignty is handed over until national elections for a transitional government are held by the end of January.
June 28, 2004: The handover of sovereignty to the interim Iraqi government takes place two days before the June 30 deadline previously announced by the U.S.-led coalition.
June 30, 2004: The coalition turns over legal control of Hussein and 11 other former top Iraqi officials to the interim Iraqi government. The U.S. retains physical custody of the men.
July 1, 2004: Hussein makes his first appearance in court. A judge charges him with a variety of crimes including the invasion of Kuwait and the gassing of the Kurds.
August 2004: U.S. and Iraqi forces battle insurgents in Najaf who are followers of Shiite cleric Muqtada al-Sadr.
September 6, 2004: The number of U.S. troops killed in Iraq reaches 1,000.
November 2004: U.S. and Iraqi forces battle insurgents in Fallujah. About 2,000 insurgents are killed. On November 14, Fallujah is declared to be liberated.
January 30, 2005: Millions of Iraqis cast ballots in the nation's first free election in half a century. Expatriates in 14 countries, including the U.S., also participate.
April 6-7, 2005: Kurdish leader Jalal Talabani is selected as the country's president by the transitional national assembly. He and two vice presidents are sworn in.
December 15, 2005: Millions of Iraqis participate in an election to choose a 275-seat Parliament that will serve a four-year term.
November 5, 2006: The Iraqi High Tribunal reaches a verdict in the 1982 Dujail massacre case. Hussein is found guilty and sentenced to death by hanging, pending appeal.
December 30, 2006: Hussein is hanged a few minutes after 6 a.m.
January 10, 2007: A troop surge begins, increasing U.S. troop levels to more than 150,000.
September 3, 2007: Basra is turned over to local authorities after British troops withdraw from their last military base in Iraq to an airport outside the city.
March 22, 2008: The number of U.S. troops killed in Iraq reaches 4,000.
July 10, 2008: Gen. David Petraeus is confirmed by the U.S. Senate as commander of U.S. Central Command. Gen. Ray Odierno succeeds Petraeus as Multinational Forces commander in Iraq.
July 16, 2008: The surge officially ends, and troop levels are reduced.
December 4, 2008 – The Iraqi Presidential Council approves a security agreement that paves the way for the U.S. to withdraw completely by 2011.
January 1, 2009: The U.S. military hands over control of Baghdad's Green Zone to Iraqi authorities.
February 27, 2009: Obama announces a date for the end of U.S. combat operations in Iraq: August 31, 2010.
June 30, 2009: U.S. troops pull back from Iraqi cities and towns, and Iraqi troops take over the responsibility for security operations. However, U.S. troops remain in the country to continue combat operations and patrols in rural areas.
August 19, 2010: The last U.S. combat brigade leaves Iraq. About 52,000 U.S. troops remain in the country.
September 1, 2010: Operation Iraqi Freedom is renamed Operation New Dawn to reflect the reduced role U.S. troops will play in securing the country.
May 22, 2011: The last British military forces in Iraq, 81 Royal Navy sailors patrolling in the Persian Gulf, withdraw from the country. During the country's eight-year mission, 179 British troops died.
October 17, 2011: A senior U.S. military official tells CNN that the U.S. and Iraq have been unable to come to agreement regarding legal immunity for U.S. troops who would remain in Iraq after the end of the year, effectively ending discussion of maintaining an American force presence after the end of 2011.
Sources: CNN, Department of Defense