The two main rivers of Iraq, the Tigris and Euphrates, have heavily influenced the development of civilization in the region and are central to the country's economy and population distribution. The United States initially became involved in Iraq in 1991 when a US-led coalition forced Saddam Hussein's Iraqi army to withdraw from Kuwait after invading. US troops remained in Iraq after 2003 to help establish a democratic government after the fall of Saddam Hussein's dictatorship and departure of US forces was not officially declared complete until December 2011.