The conflict continued for much of the next ten years as an insurgency emerged to oppose the occupying forces and the post-invasion Iraqi government. An estimated 151,000 to 600,000 or more Iraqis were killed in the first 3–4 years of conflict. The United States officially withdrew from the country in 2011 but left private security contractors in its place to continue the war. In 2014 the Islamic State of Iraq and the Levant conquered much of northern Iraq. An American-led new coalition sent troops to help the government of Iraq.