The invasion began in 2003 when the United States , joined by the United Kingdom and several coalition allies , launched a " shock and awe " surprise attack without declaring war . Iraqi forces were quickly overwhelmed as U.S. forces swept throughout the country . The invasion led to the collapse of the Ba'athist government ; Saddam was captured , and he was executed by a military court three years later . However , the power vacuum following Saddam 's fall , the mismanagement of the occupation and the sectarian policies of various militias led to a lengthy insurgency against U.S. , coalition forces and Iraqi government forces as well as widespread sectarian violence between Shias and Sunnis . The United States responded with a troop surge in 2007 ; the heavy American security presence and deals made between the occupying forces and Sunni militias reduced the level of violence . The U.S. began withdrawing its troops in the winter of 2007 – 2008 . The winding down of U.S. involvement in Iraq accelerated under President Barack Obama . The U.S. withdrew all combat troops from Iraq by 2011
Saddam was captured
September 11 attacks
Iraqi insurgency (2011–2013)