The Iraq War[nb 1] 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.[55] An estimated 151,000 to 1,033,000 Iraqis were killed in the first three to four years of conflict. US troops were officially withdrawn in 2011. However, following the spread of the Syrian Civil War and the territorial gains of the Islamic State of Iraq and the Levant (ISIL), the Obama administration decided to redeploy US forces to Iraq in 2014. Many former soldiers are employed by defence contractors and private military companies.[56][57] The U.S. 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 the George W. Bush administration's War on Terror, following the September 11 attacks.[58]

In October 2002, Congress authorized President Bush to launch a military attack against Iraq if he decides it is necessary.[59] The Iraq War began on 20 March 2003,[60] when the U.S., joined by the U.K. and several coalition allies, launched a "shock and awe" bombing campaign. Iraqi forces were quickly overwhelmed as coalition forces swept through the country. The invasion led to the collapse of the Ba'athist government; Saddam Hussein was captured during Operation Red Dawn in December of that same year and executed three years later. The power vacuum following Saddam's demise and the mismanagement of the Coalition Provisional Authority led to widespread civil war between Shias and Sunnis, as well as a lengthy insurgency against coalition forces. Many of the violent insurgent groups were supported by Iran and al-Qaeda in Iraq. The United States responded with a build-up of 170,000 troops in 2007.[61] This build-up gave greater control to Iraq's government and military, and was judged a success by many.[62] In 2008, President Bush agreed to a withdrawal of all U.S. combat troops from Iraq. The withdrawal was completed under President Barack Obama in December 2011.[63][64]
The Bush administration based its rationale for the Iraq War principally on the assertion that Iraq, which had been viewed by the U.S. as a "rogue state" since the 1990–1991 Gulf War, supposedly possessed an active weapons of mass destruction (WMD) program,[65] and that the Iraqi government posed a threat to the United States and its coalition allies.[66][67] Some U.S. officials falsely accused Saddam of harbouring and supporting al-Qaeda,[68] while others cited the desire to end a repressive dictatorship and bring democracy to the people of Iraq.[69][70] In 2004, the 9/11 Commission said there was no evidence of an operational relationship between the Saddam Hussein regime and al-Qaeda.[71] No stockpiles of WMDs or an active WMD program were ever found in Iraq.[72] Bush administration officials made numerous assertions about a purported Saddam-al-Qaeda relationship and WMDs that were based on sketchy evidence, and which intelligence officials rejected.[72][73] The rationale of U.S. pre-war intelligence faced heavy criticism both domestically and internationally.[74] The Chilcot Report, a British inquiry into its decision to go to war, was published in 2016 and concluded military action may have been necessary but was not the last resort at the time and that the consequences of invasion were underestimated.[75] When interrogated by the FBI, Saddam Hussein admitted to having kept up the appearance of possessing weapons of mass destruction in