The United States formally declares an end to the Iraq War. While this ends the insurgency, it begins another.

An insurgency began in Iraq after the 2003 US-led invasion, and lasted throughout the ensuing Iraq War (2003–2011). The first phase of the insurgency began shortly after the 2003 invasion and prior to the establishment of the new Iraqi government. From around 2004 to May 2007, the insurgency primarily targeted the Multi-National Force – Iraq, while latterly, Iraqi security forces, seen, by Iraqi insurgents, as collaborators with the coalition, were also targeted.

With the full-scale eruption of the sectarian civil war in February 2006, many militant attacks in American-controlled central Iraq were directed at the Iraqi police and military forces of the Iraqi government. The attacks continued during the transitional reconstruction of Iraq, as the Iraqi government tried to establish itself. Civil war violence decreased in late 2008 and the insurgency continued through the American withdrawal from Iraq in 2011. After the withdrawal in December 2011, a renewed wave of sectarian and anti-government insurgency swept Iraq, causing thousands of casualties in 2012. Increasing violence in 2013 raised fears of another civil war.

The insurgents in Iraq have been composed of a diverse mix of militias, foreign fighters, all-Iraqi units or mixtures opposing the American-led Multi-National Force – Iraq and the post-2003 Iraqi government. During the height of the Iraq War in 2006 to 2008, the fighting involved both armed conflict against the American-led military coalition, as well as sectarian violence among the different ethnic groups within the population. The insurgents were involved in asymmetric warfare and a war of attrition against the American-supported Iraqi government and American forces in central Iraq, while conducting coercive tactics against rivals or other militias. Iraq's deep sectarian divides have been a major dynamic in the insurgency, with support for the insurgents varying among different segments of the population.