The Iraq War began on March 20, 2003 with the invasion of Iraq by the United States under the administration of President George W. Bush and the United Kingdom under Prime Minister Tony Blair. The war is also referred to as the Second Gulf War or Operation Iraqi Freedom.

Prior to the invasion, the governments of the United States and the United Kingdom asserted that the possibility of Iraq employing weapons of mass destruction (WMD) threatened their security and that of their coalition/regional allies.

In 2002, the United Nations Security Council passed Resolution 1441, which called for Iraq to completely cooperate with UN weapon inspectors to verify that it was not in possession of weapons of mass destruction. The United Nations Monitoring, Verification, and Inspection Commission (UNMOVIC) was given access by Iraq under provisions of the UN resolution.

Additional months of inspection to conclusively verify Iraq's compliance with the UN disarmament requirements were not undertaken. Head weapons inspector Hans Blix advised the UN Security Council that while Iraq's cooperation was "active," it was not "unconditional" and not "immediate." Iraq's declarations with regards to weapons of mass destruction could not be verified at the time, but unresolved tasks concerning Iraq's disarmament could be completed in "not years, not weeks, but months."

Following the invasion, the US-led Iraq Survey Group concluded that Iraq had ended its nuclear, chemical, and biological programs in 1991 and had no active programs at the time of the invasion, but that Iraq intended to resume production once sanctions were lifted. Although some degraded remnants of misplaced or abandoned chemical weapons from before 1991 were found, they were not the weapons which had been the main issue. Some US officials also accused Iraqi President Saddam Hussein of harboring and supporting al-Qaeda. Other reasons for the invasion given by the governments of the attacking countries included Iraq's financial support for the families of Palestinian suicide bombers and Iraqi government human rights abuses.

The invasion of Iraq led to the eventual capture of President Saddam, who was later tried in an Iraqi court of law and executed by the new Iraqi government. Violence against coalition forces and among various sectarian groups soon led to the Iraqi insurgency, strife between many Sunni and Shia Iraqi groups, and the emergence of a new faction of al-Qaeda in Iraq. 

In 2008, the UN reported an estimate of 4.7 million refugees with 2 million abroad (a number close to CIA projections) and 2.7 million internally displaced people.

In June 2008, U.S. Department of Defense officials claimed security and economic indicators began to show signs of improvement in what they hailed as significant and fragile gains.

In 2007, Iraq was second on the Failed States Index; though its ranking has steadily improved since then, moving to fifth on the 2008 list, sixth in 2009, and seventh in 2010. As public opinion favoring troop withdrawals increased and as Iraqi forces began to take responsibility for security, member nations of the Coalition withdrew their forces. In late 2008, the US and Iraqi governments approved a Status of Forces Agreement effective through 1 January 2012. The Iraqi Parliament also ratified a Strategic Framework Agreement with the US, aimed at ensuring cooperation in constitutional rights, threat deterrence, education, energy development, and other areas.
 
In late February 2009, newly elected US President Barack Obama announced an 18-month withdrawal window for combat forces, with approximately 50,000 troops remaining in the country "to advise and train Iraqi security forces and to provide intelligence and surveillance." General Ray Odierno, the top US military commander in Iraq, said he believes all US troops will be out of the country by the end of 2011, whereas UK forces ended combat operations on 30 April 2009.

Iraqi Prime Minister Nouri al-Maliki has said he supports the accelerated pullout of US forces. In a speech at the Oval Office on 31 August 2010 Obama declared "the American combat mission in Iraq has ended. Operation Iraqi Freedom is over, and the Iraqi people now have lead responsibility for the security of their country."

Beginning 1 September 2010, the American operational name for its involvement in Iraq changed from "Operation Iraqi Freedom" to "Operation New Dawn." The remaining 50,000 US troops are now designated to "advise and assist brigades" assigned to non-combat operations while retaining the ability to revert to combat operations as necessary. Two combat aviation brigades also remain in Iraq.

Casualty counts for the conflict in Iraq since 2003, beginning with the 2003 invasion of Iraq and continuing with the ensuing occupation of Iraq coalition presence, as well as the activities of the various armed groups operating in the country, come in many forms.

For troops in the US-led multinational coalition, the death toll is as follows:
US named dead – 4,282
US wounded – 30,182

Non-Combat Deaths:
Iraq Family Health Survey – 151,000 deaths
Iraq Body Count Project – 102,417 to 111,938 civilian deaths as a result of combat operations

The word "casualties" in its most general sense includes the injured as well as the dead.