The Mahdi Army—a Shia militia created in the summer of 2003 by Muqtada al-Sadr—began to fight Coalition forces in April 2004. 2004 saw Sunni and Shia militants fighting against each other and against the new Iraqi Interim Government installed in June 2004, and against Coalition forces, as well as the First Battle of Fallujah in April and Second Battle of Fallujah in November. The Sunni militia Jama’at al-Tawhid wal-Jihad became Al-Qaeda in Iraq in October 2004 and targeted Coalition forces as well as civilians, mainly Shia Muslims, further exacerbating ethnic tensions.
In January 2005, the first elections since the invasion took place and in October a new Constitution was approved, which was followed by parliamentary elections in December. However, insurgent attacks were common and increased to 34,131 in 2005 from 26,496 in 2004.
During 2006, fighting continued and reached its highest levels of violence, more war crimes scandals were made public, Abu Musab al-Zarqawi the leader of Al-Qaeda in Iraq was killed by US forces and Iraq’s former dictator Saddam Hussein was sentenced to death for crimes against humanity and hanged.
In late 2006, the US government’s Iraq Study Group recommended that the US begin focusing on training Iraqi military personnel and in January 2007 US President George W. Bush announced a “Surge” in the number of US troops deployed to the country.
In May 2007, Iraq’s Parliament called on the United States to set a timetable for withdrawal and US coalition partners such as the UK and Denmark began withdrawing their forces from the country. The war in Iraq has resulted in between 151,000 and 1.2 million Iraqis being killed.
In 2008, fighting continued and Iraq’s newly trained armed forces launched attacks against militants. The Iraqi government signed the US–Iraq Status of Forces Agreement, which required US forces to withdraw from Iraqi cities by 30 June 2009 and to withdraw completely from Iraq by 31 December 2011.
US troops handed over security duties to Iraqi forces in June 2009, though they continued to work with Iraqi forces after the pullout. On the morning of 18 December 2011, the final contingent of US troops to be withdrawn ceremonially exited over the border to Kuwait.
Crime and violence initially spiked in the months following the US withdrawal from cities in mid-2009 but despite the initial increase in violence, in November 2009, Iraqi Interior Ministry officials reported that the civilian death toll in Iraq fell to its lowest level since the 2003 invasion.
Following the withdrawal of US troops in 2011, the insurgency continued and Iraq suffered from political instability. In February 2011, the Arab Spring protests spread to Iraq; but the initial protests did not topple the government. The Iraqi National Movement, reportedly representing the majority of Iraqi Sunnis, boycotted Parliament for several weeks in late 2011 and early 2012, claiming that the Shiite-dominated government was striving to sideline Sunnis.