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The Iraq War (Arabic: حرب العراق, romanized: ḥarb al-ʿirāq), sometimes called the Second Gulf War, was a protracted armed conflict in Iraq from 2003 to 2011. It began with the invasion of Iraq by the United States-led coalition that overthrew the Ba'athist 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 US troops were officially withdrawn in 2011.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. 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 in 2001 in the United States.

In October 2002, the United States Congress passed a joint resolution that granted Bush the power to use military force against the Iraqi government. The Iraq War officially began on 20 March 2003, when the US, joined by the United Kingdom, Australia, and Poland, launched a " shock and awe " bombing campaign. Shortly following the bombing campaign, US-led forces launched a ground invasion of Iraq. 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 while also giving the United States a greater say in the postwar reconstruction of Iraq. 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 that Saddam Hussein was supporting al-Qaeda. The US government had also alleged that Al-Qaeda was secretly co-operating with Iraq to build weapons of mass destruction and argued that Iraq posed a threat to the United States and its allies. However, in 2004, the 9/11 Commission concluded that 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 had also 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 the Iraq 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 had 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, although the Iraq Survey Group did find that Saddam had the aim of WMD proliferation and maintained the laboratories and scientists necessary for WMD development.

In 2005, Iraq held multi-party elections. 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.

The war killed an estimated 150,000 to 1,033,000 people, including more than 100,000 civilians (see estimates below). Most died during the initial insurgency and civil conflicts. The 2013–2017 War in Iraq, which is considered a domino effect of the invasion and occupation, caused at least 155,000 deaths and internally displaced more than 3.3 million Iraqis.

The war hurt the United States' international reputation as well as Bush's domestic popularity and public image. It also reduced UK prime minister Tony Blair 's popularity, leading to his resignation in 2007.