Sheikh Sabah al-Ahmad al-Sabah, Emir of Kuwait, has died at 91.
He was flown to the United States for medical treatment following surgery for an unspecified condition in Kuwait in July.
The Emir ruled the oil-rich gulf state since 2006 and was in charge of its foreign policy for more than 50 years.
He was popularly called "Dean of Arab diplomacy" for his efforts to restore relations with states that backed Iraq during the 1990-1991 Gulf War when Kuwait was invaded by Iraqi forces.
He was a mediator in regional disputes including the ongoing diplomatic stand-off between Saudi Arabia, its allies and Qatar.
He also had the last say in political matters and had the powers to override or dissolve parliament and call elections.
Sheikh Sabah came to power in January 2006, after Emir Sheikh Saad al-Abdullah stepped down just nine days into his rule as parliament moved to depose him on health grounds.
He had been Prime Minister under the previous Emir, Sheikh Jaber al-Ahmad al-Jaber al-Sabah, and for several years had been seen as the de facto ruler.