An insurgency began in Iraq after the 2003 U.S.-led invasion, and lasted throughout the ensuing Iraq War which lasted from 2003 until 2011. The first phase of the insurgency began shortly after the 2003 invasion and before the establishment of the new Iraqi government. From around 2004 to May 2007, Iraqi insurgents primarily targeted the American-led coalition forces, and later also targeted Iraqi security forces. 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 and sectarian violence ended in mid-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 led to the War in Iraq (2013–2017). 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. 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 had been a major dynamic in the insurgency, with support for the insurgents varying among different segments of the population. Background The 2003 invasion of Iraq (19 March – 1 May 2003) began the Iraq War, or Operation Iraqi Freedom, in which a combined force of troops from the United States, the United Kingdom, Australia, and Poland invaded Iraq and toppled the government of Saddam Hussein within 21 days of major combat operations. The invasion phase consisted of a conventionally fought war which concluded with the capture of the Iraq capital Baghdad by U.S. forces. Four countries participated with troops during the initial invasion phase, which lasted from 19 March to 9 April 2003. These were the United States (148,000), United Kingdom (45,000), Australia (2,000), and Poland (194). Thirty-six other countries were involved in