The Iraq War was a protracted armed conflict in Iraq from 2003 to 2011 that began with the invasion of Iraq by the United States-led coalition that overthrew the Iraqi government of Saddam Hussein. The conflict continued for much of the next decade as an insurgency emerged to oppose the coalition forces and the post-invasion Iraqi government. US troops were officially withdrawn in 2011. The United States became re-involved in 2014 at the head of a new coalition, and the insurgency and many dimensions of the armed conflict are ongoing. The invasion occurred as part of the George W. Bush administration's war on terror following the September 11 attacks, despite no connection between Iraq and the attacks.
In October 2002, Congress granted Bush the power to decide whether to launch any military attack in Iraq. The Iraq War began on 20 March 2003, when the US, joined by the United Kingdom, Australia, and Poland, 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 mismanagement by the Coalition Provisional Authority, led to widespread civil war between Shias and Sunnis, as well as a lengthy insurgency against coalition forces. The United States responded with a build-up of 170,000 troops in 2007. This build-up gave greater control to Iraq's government and military, and was judged a success by many. In 2008, President Bush agreed to a withdrawal of all US combat troops from Iraq. The withdrawal was completed under Barack Obama in December 2011.
The United States based most of its rationale for the invasion on claims that Iraq had a weapons of mass destruction (WMD) program and posed a threat to the United States and its allies. Additionally, some US officials accused Saddam of harbouring and supporting al-Qaeda. In 2004, the 9/11 Commission concluded there was no evidence of any relationship between Saddam's regime and al-Qaeda. No stockpiles of WMDs or active WMD program were ever found in Iraq. Bush administration officials made numerous claims about a purported Saddam–al-Qaeda relationship and WMDs that were based on insufficient evidence rejected by intelligence officials. The rationale for war faced heavy criticism both domestically and internationally. Kofi Annan, then the Secretary-General of the United Nations, called the invasion illegal under international law, as it violated the UN Charter. The 2016 Chilcot Report, a British inquiry into the United Kingdom's decision to go to war, concluded that not every peaceful alternative had been examined, that the UK and US had undermined the United Nations Security Council in the process of declaring war, that the process of identification for a legal basis of war was "far from satisfactory", and that, these conclusions taken together, the war was unnecessary. When interrogated by the FBI, Saddam Hussein confirmed that Iraq did not have weapons of mass destruction prior to the US invasion.
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 alienated the country's previously dominant Sunni minority and worsened sectarian tensions. In the summer of 2014, the Islamic State launched a military offensive in northern Iraq and declared a worldwide Islamic caliphate, leading to Operation Inherent Resolve, another military response from the United States and its allies. According to a 2019 US Army study, Iran emerged as "the only victor" of the war.
An estimated 151,000 to 1,033,000 Iraqis died in the first three to five years of conflict. In total, the war caused 100,000 or more civilian deaths - about 61% of the total death count - as well as 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. Subsequently, the War in Iraq of 2013 to 2017, which is considered a domino effect of the invasion and occupation, caused at least 155,000 deaths, in addition to the displacement of more than 3.3 million people within the country. Additionally, the war hampered the domestic popularity and public image of Bush, and also strongly affected Blair's popularity in the United Kingdom, leading to his resignation in 2007.