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.