The Iraq War was a protracted armed conflict that began in 2003 with the invasion of Iraq by a United States-led coalition that overthrew the government of Saddam Hussein. The conflict continued for much of the next decade as an insurgency emerged to oppose the occupying forces and the post-invasion Iraqi government. An estimated 151,000 to 600,000 or more Iraqis were killed in the first 3–4 years of conflict. The US became re-involved in 2014 at the head of a new coalition; the insurgency and many dimensions of the civil armed conflict continue. The invasion occurred as part of a declared war against international terrorism and its sponsors under the administration of US President George W. Bush following the September 11 terror attacks.
The invasion began on 20 March 2003, with the US, joined by the United Kingdom and several coalition allies, launching a "shock and awe" bombing campaign. 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 during Operation Red Dawn in December of that same year 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. Many violent insurgent groups were supported by Iran and al-Qaeda in Iraq. The United States responded with a troop surge in 2007. The winding down of U.S. involvement in Iraq accelerated under President Barack Obama. The US formally withdrew all combat troops from Iraq by December 2011.
The Bush administration based its rationale for the war principally on the assertion that Iraq, which had been viewed by the US as a rogue state since the Persian Gulf War, possessed weapons of mass destruction (WMDs) and that the Iraqi government posed an immediate threat to the United States and its coalition allies. Select U.S. officials accused Saddam of harbouring 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, while claims of Iraqi officials collaborating with al-Qaeda were proven false. The rationale and misrepresentation of US prewar intelligence faced heavy criticism both domestically and internationally, with President Bush declining from his record-high approval ratings following 9/11 to become one of the most unpopular presidents in US history.
In the aftermath of the invasion, Iraq held multi-party elections in 2005. Nouri al-Maliki became Prime Minister in 2006 and remained in office until 2014. The al-Maliki government enacted policies that were widely seen as having the effect of alienating the country's Sunni minority and worsening 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 over a hundred thousand civilian deaths and tens of thousands of military deaths (see estimates below). The majority of deaths occurred as a result of the insurgency and civil conflicts between 2004 and 2007.