 Complete the following phrase:  Reported acts of violence conducted by an uneasy tapestry of insurgents steadily increased by the end of 2006. Sunni jihadist forces including Al Qaeda in Iraq continued to target Shia civilians, notably in the 23 February 2006 attack on the Al Askari Mosque in Samarra, one of Shiite Islams holiest sites leading to a civil war between Sunni and Shia militants in Iraq. Analysis of the attack suggested that the Mujahideen Shura Council and Al-Qaeda in Iraq were responsible, and that the motivation was to provoke further violence by outraging the Shia population. In mid-October 2006, a statement was released stating that the Mujahideen Shura Council had been disbanded and was replaced by the Islamic State of Iraq. It was formed to resist efforts by the U.S. and Iraqi authorities to win over Sunni supporters of the insurgency. Shia militias, some of whom were associated with elements in the Iraq government, reacted with reprisal acts against the Sunni minority. A cycle of violence thus ensued whereby Sunni insurgent attacks were followed reprisals by Shiite militias, often in the form of Shiite death squads that sought out and killed Sunnis. Following a surge in U.S. troops in 2007 and 2008, violence in Iraq began to decrease. The U.S. ended their main military presence in 2011, however, resulting in 