On the Tigris and Euphrates rivers and once known as Mesopotamia, Iraq was the site of flourishing ancient civilizations. Arabs invaded the region in the 7th century and brought the Muslim religion, and for a time Iraq was the heartland of the Islamic Empire. The 13th century Mongol invasion followed by Ottoman Turks, and Iraq became part of Ottoman Empire from the 15th century. It came under British control after World War I, and gained independence in 1932. After a military coup that overthrew the monarchy, Iraq was declared a republic in 1958. Territorial disputes with Iran led to a costly eight-year war (1980-88). In August 1990, Iraq seized Kuwait but was expelled by United States-led, United Nations coalition forces during the Gulf War in early 1991. Following Kuwait's liberation, the United Nations Security Council required Iraq to surrender all weapons of mass destruction and long-range missiles and to allow United Nations verification inspections. Claims of continued Iraqi noncompliance with United Nations Security Council resolutions over a period of 12 years led to the United States-led justification for the invasion of Iraq in March 2003 and the ouster of the Saddam Hussein regime. Of course, the actual legal basis for such claims remains a matter of debate. In October 2005, Iraqis approved a constitution in a national referendum and elected a Council of Representatives (CoR) in December 2005. The CoR approved most cabinet ministers in May 2006, marking the transition to a constitutional government. Iraq’s economy is dominated by the oil sector. The country has the world’s third-largest oil reserves and considerable gas reserves. Although it has improved considerably, the security situation remains volatile in Iraq. Remaining United States troops in Iraq were scheduled to be fully withdraw by the end of 2011.