The Iraq War is a protracted armed conflict that began with the 2003 invasion of Iraq led by the United States. The invasion toppled the government of Saddam Hussein. The conflict, however, continued for much of the next decade as an insurgency emerged to oppose the occupying forces and the post-invasion Iraqi government. The United States officially withdrew from the country in 2011, but the insurgency and various dimensions of the civil armed conflict continue.
The invasion began on 20 March 2003. The U.S., joined by the United Kingdom and several coalition allies, launched a "shock and awe" surprise attack without declaring war. Iraqi forces were quickly overwhelmed as U.S. forces swept through the country. The invasion led to the collapse of the Ba'athist government; Saddam was captured in December 2003 and executed by a military court three years later. However, the power vacuum following Saddam's demise and the mismanagement of the occupation led to widespread sectarian violence between Shias and Sunnis as well as a lengthy insurgency against U.S. and coalition forces. The United States responded with a troop surge in 2007; the heavy American security presence and deals made between the occupying forces and Sunni militias reduced the level of violence. The U.S. began withdrawing its troops in the winter of 2007-2008. The winding down of U.S. involvement in Iraq accelerated under President Barack Obama. The U.S. formally withdrew all combat troops from Iraq by December 2011.
The Bush Administration based its rationale for war principally on the assertion that Iraq possessed weapons of mass destruction (WMDs) and that Saddam's government posed an immediate threat to the United States and its coalition allies. Select U.S. officials accused Saddam of harboring and supporting al-Qaeda, while others cited the desire to end a repressive dictatorship and bring democracy to the people of Iraq. After the invasion, no substantial evidence was found to verify the initial claims about WMDs. The rationale and misrepresentation of pre-war intelligence faced heavy criticism within the U.S. and internationally.
As a result of the war, Iraq held multi-party elections in 2005. Nouri al-Maliki became Prime Minister in 2006 and remained in office until 2014. The Maliki government enacted policies that was widely seen as having the effect of alienating the country's Sunni minority, which worsened sectarian tensions. In the summer of 2014, the Islamic State of Iraq and the Levant (ISIL) launched a military offensive in Northern Iraq and declared a worldwide Islamic caliphate, eliciting another military response from the United States and its allies. The Iraq War caused hundreds of thousands of civilian and military casualties (see estimates below). The majority of casualties occurred as a result of the insurgency and civil conflicts between 2004 and 2007.