Here's a look at what you need to know about the Iraq War which was known as Operation Iraqi Freedom until September 2010, when it was renamed Operation New Dawn.
Timeline: 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 President Hussein and his family - leave Iraq within 48 hours or face military action.
March 19, 2003 (10:15 p.m. EST) - President Bush announces U.S. and coalition forces have begun military action against Iraq.
March 20, 2003 - President Hussein speaks on Iraqi TV, calling the coalition's attacks "shameful crimes against Iraq and humanity."
March 23, 2003 - Members of the 507th Maintenance Company are ambushed and captured outside Nasiriyah, Iraq.
April 1, 2003 - Pfc. Jessica Lynch is rescued from a hospital by U.S. forces.
April 9, 2003 - Coalition forces take Baghdad. A large statue of Saddam Hussein is toppled down 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 - Speaking on the U.S.S. Abraham Lincoln, President 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 Great Britain's right to occupy Iraq.
July 22, 2003 - Saddam Hussein's sons, Uday and Qusay, are killed by U.S. forces.
December 13, 2003 - Saddam Hussein is captured in a "spider hole" in Tikrit. This is not confirmed until December 14 by the U.S. Defense Dept.
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 Saddam Hussein and 11 other former top Iraqi officials to the interim Iraqi government. The United States retains physical custody of the men.
July 1, 2004 - Saddam Hussein makes his first appearance in court. A judge charges Hussein 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 Falluja. About 2,000 insurgents are killed. On November 14, Falluja is declared to be liberated.
October 25, 2005 - The number of U.S. troops killed in Iraq reaches 2,000.
November 19, 2005 - At least 24 Iraqi civilians, including women and children, are killed in Haditha, Iraq. Suspicion falls on Marines from Kilo Company of the 3rd Battalion, 5th Marine Regiment. Eight U.S. Marines faced charges in the deaths, but only one is convicted of a crime, that of negligent dereliction of duty.
November 5, 2006 - The Iraqi High Tribunal reaches a verdict in the 1982 Dujail massacre case. Saddam Hussein is found guilty and sentenced to death by hanging, pending appeal.
December 30, 2006 - Saddam Hussein is hanged a few minutes after 6 a.m. Baghdad time (10 p.m. EST).
December 30, 2006 - The number of U.S. troops killed in Iraq reaches 3,000.
January 10, 2007 - A troop surge begins, eventually 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 U.S. Senate as commander of U.S. Central Command (CENTCOM). General 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 United States to withdraw completely from Iraq by 2011.
January 1, 2009 - The U.S. military hands over control of Baghdad's Green Zone to Iraqi authorities.
February 27, 2009 - President Barack 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. 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. 179 British troops died during the country's eight-year mission in Iraq.
October 17, 2011 - A senior U.S. military official tells CNN that the United STates 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.
October 21, 2011 - President Barack Obama announces that virtually all U.S. troops will come home from Iraq by the end of the year. According to a U.S. official, about 150 of the 39,000 troops currently in Iraq will remain to assist in arms sales. The rest will be out of Iraq by December 31.
December 2, 2011 - The last U.S. troops to occupy Camp Victory leave as the Iraqi government assumes control of the complex.
December 15, 2011 - American troops lower the flag of command that flies over Baghdad officially ending the U.S. military mission in Iraq.
December 18, 2011 - The last U.S. troops in Iraq cross the border into Kuwait.