Cambodia (; also Kampuchea ; , UNGEGN: ), officially the Kingdom of Cambodia, is a country located in the southern portion of the Indochinese Peninsula in Southeast Asia, spanning an area of , bordered by Thailand to the northwest, Laos to the north, Vietnam to the east, and the Gulf of Thailand to the southwest. The capital and largest city is Phnom Penh. The sovereign state of Cambodia has a population of over 17 million. Buddhism is enshrined in the constitution as the official state religion, and is practised by more than 97% of the population. Cambodia's minority groups include Vietnamese, Chinese, Chams and 30 hill tribes. Cambodia has a tropical monsoon climate of two seasons, and the country is made up of a central floodplain around the Tonlé Sap lake and Mekong Delta, surrounded by mountainous regions. The capital and largest city is Phnom Penh, the political, economic and cultural centre of Cambodia. The kingdom is an elective constitutional monarchy with a monarch, currently Norodom Sihamoni, chosen by the Royal Council of the Throne as head of state. The head of government is the Prime Minister, currently Hun Sen, the longest serving non-royal leader in Southeast Asia, who has ruled since 1985. The region now known as Cambodia has been inhabited since prehistoric times. In 802 AD, Jayavarman II declared himself king, uniting the warring Khmer princes of Chenla under the name "Kambuja". This marked the beginning of the Khmer Empire, which flourished for over 600 years. The Indianised kingdom facilitated the spread of first Hinduism and then Buddhism to much of Southeast Asia and undertook many religious infrastructural projects throughout the region. Angkor Wat is the most famous of these structures and is designated as a World Heritage Site. In the fifteenth century, Cambodia experienced a decline of power, while its neighbors Vietnam and Thailand grew stronger. In 1863, Cambodia became a protectorate of France, and later was part of French Indochina. After a period of Japanese occupation during the Second World War, Cambodia gained independence from France in 1953. Despite Cambodia's neutrality, the Vietnam War extended into the country in 1965 via the Ho Chi Minh and Sihanouk trails. A 1970 coup installed the US-aligned Khmer Republic, until being overthrown by the Khmer Rouge in 1975. The Khmer Rouge ruled the country and carried out the Cambodian genocide from 1975 until 1979, when they were ousted in