United States invades Iraq

On March 19, 2003, President George W. Bush announced that American forces had begun military operations against Iraq, and on March 20, 2003 U.S. and coalition troops crossed from Kuwait into Iraqi territory. Bush, Defense Secretary Donald Rumsfeld, and coalition commanders justified the invasion by citing Saddam Hussein's alleged weapons of mass destruction, Iraq's defiance of United Nations resolutions, and the administration's post-September 11 security doctrine. British prime minister Tony Blair joined the United States in the opening assault, while Baghdad became the main political and military objective of the campaign.

The invasion intensified the dispute over whether preventive war and regime change were legitimate tools of American foreign policy after the September 11 attacks. The Bush administration argued that the Authorization for Use of Military Force Against Iraq Resolution of 2002 and earlier U.N. resolutions provided enough legal and political basis for action, while France, Germany, Russia, and many critics in the United States argued that Washington had not proved an immediate threat. The operation also exposed the tension between rapid battlefield victory and the much harder problem of governing Iraq after Saddam Hussein's state collapsed.

The fall of Baghdad on April 9, 2003 and the establishment of the Coalition Provisional Authority followed directly from the invasion. The long insurgency that emerged in 2003, the Iraq troop surge of 2007, and the later rise of the Islamic State all grew out of the occupation that began when the United States invaded Iraq.