The Iraq War was a protracted armed conflict that began in 2003 with the invasion of Iraq by a United States-led coalition that overthrew the government of Saddam Hussein. The conflict continued for much of the next decade 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 three to four years of conflict. In 2009, official US troops were withdrawn, but American soldiers continued to remain on the ground fighting in Iraq, retained by defence contractors and private military companies. The U.S. became re-involved in 2014 at the head of a new coalition. The invasion occurred as part of a declared war against international terrorism and its sponsors under the administration of U.S. President George W. Bush following the September 11, 2001 terrorist attacks on the US.