The Iraq War, also known as the Second Gulf War, the Occupation of Iraq, or the War in Iraq, is an ongoing military campaign which began on March 20, 2003 with the 2003 invasion of Iraq by a multinational coalition composed of United States and United Kingdom troops supported by smaller contingents from Australia, Denmark, Poland and other nations.

Prior to the war, Iraq's alleged possession of weapons of mass destruction (WMD) were claimed to pose a serious and imminent threat to Western national security. This assessment was supported by the U.K. intelligence services, but not by other countries such as France, Russia and Germany. United Nations Monitoring, Verification and Inspection Commission weapon inspectors found no evidence of WMD, giving support to earlier criticism of poor intelligence on Iraqi WMDs. After the invasion, the U.S.-led Iraq Survey Group concluded that Iraq had ended its WMD programs in 1991 and had no active programs at the time of the invasion, but that they intended to resume production if the Iraq sanctions were lifted. Although some degraded remnants of misplaced or abandoned chemical weapons from before 1991 were found, they were not the weapons for which the coalition invaded. The failure to find WMD in Iraq caused controversy, particularly in the United States. Some U.S. officials also accused Saddam Hussein of harboring and supporting Al-Qaeda, but no evidence of any collaborative relationship was found. Other reasons for the invasion stated by U.S. officials included Iraq's alleged financial support for the families of Palestinian suicide bombers, Iraqi government human rights abuses, spreading democracy, and Iraq's oil reserves, although the latter was denied by other officials. Bush said either that God inspired him to end the tyranny in Iraq, or to hit Saddam.

The invasion led to the quick defeat of the Iraqi military, and the capture and execution of Saddam Hussein. The U.S.-led coalition occupied Iraq and attempted to establish a new democratic government; however, violence against coalition forces and among various sectarian groups soon led to asymmetric warfare with the Iraqi insurgency, strife between many Sunni and Shia Iraqi groups, and al-Qaeda operations in Iraq. Estimates of the number of Iraqis killed through 2007 range from 150,000 to more than 1,000,000. UNHCR estimates the war created 4.7 million Iraqi refugees through April 2008 (~16% of the population of Iraq.) UNHCR - Iraq: Latest return survey shows few intending to go home soon. Published April 29, 2008. Retrieved May 20, 2008. The U.S. Department of Defense claimed in 2008 that "the security, political and economic trends in Iraq continue to be positive, however, they remain fragile, reversible and uneven." Iraq was fifth on the 2008 Failed States Index, and the Red Cross stated in 2008 that Iraq's humanitarian situation "remains among the most critical in the world". Member nations of the Coalition began to withdraw their forces as public opinion favoring troop withdrawals increased in their countries and Iraq and as Iraqi forces began to take responsibility for security. U.S. and Iraqi officials are debating the timeline and magnitude of an American withdrawal, with Iraqi Prime Minister Nouri al-Maliki pushing for a complete withdrawal of U.S. forces from Iraq by 2011.