The Funan empire was established in what is now Cambodia in the 1st cent. a.d. By the 3d cent. the Funanese, under the leadership of Fan Shih-man (reigned 205-25), had conquered their neighbors and extended their sway to the lower Mekong River. In the 4th cent., according to Chinese records, an Indian Brahmin extended his rule over Funan, introducing Hindu customs, the Indian legal code, and the alphabet of central India.

In the 6th cent. Khmers from the rival Chen-la state to the north overran Funan. With the rise of the Khmer Empire, Cambodia became dominant in SE Asia. Angkor, the capital of the Khmer empire, was one of the world's great architectural achievements. After the fall of the empire (15th cent.), however, Cambodia was the prey of stronger neighbors. To pressure from Siam on the western frontier was added in the 17th cent. pressure from Annam on the east; the kings of Siam and the lords of Hue alike asserted overlordship and claims to tribute. In the 18th cent. Cambodia lost three western provinces to Siam and the region of Cochin China to the Annamese.

Intrigue and wars on Cambodian soil continued into the 19th cent., and in 1854 the king of Cambodia appealed for French intervention. A French protectorate was formally established in 1863, and French influence was consolidated by a treaty in 1884. Cambodia became part of the Union of Indochina in 1887. In 1907 a French-Siamese treaty restored Cambodia's western provinces. In World War II, under Japanese occupation, Cambodia again briefly lost those provinces to Siam.

In Jan., 1946, France granted Cambodia self-government within the French Union; a constitution was promulgated in May, 1947. A treaty signed in 1949 raised the country's status to that of an associated state in the French Union, but limitations on the country's sovereignty persisted. King Norodom Sihanouk campaigned for complete independence, which was finally granted in 1953. Early in 1954, Communist Viet Minh troops from Vietnam invaded Cambodia. The Geneva Conference of 1954 led to an armistice providing for the withdrawal of all foreign forces from Cambodia. An agreement between France and Cambodia (Dec., 1954) severed the last vestige of French control over Cambodian policy. Cambodia withdrew from the French Union in 1955 and was admitted into the United Nations later that year.

King Norodom Sihanouk abdicated in Mar., 1955, in order to enter politics; his father, Norodom Suramarit, succeeded him as monarch. Sihanouk subsequently formed the Popular Socialist party and served as premier. After Suramarit's death in 1960, the monarchy was represented by Sihanouk's mother, Queen Kossamak Nearireak. Sihanouk was installed in the new office of chief of state. Throughout the 1960s, Sihanouk struggled to keep Cambodia neutral as the neighboring countries of Laos and South Vietnam came under increasing Communist attack (see Vietnam War). Sihanouk permitted the use of Cambodian territory as a supply base and refuge by North Vietnamese and Viet Cong troops while accepting military aid from the United States to strengthen his forces against Communist infiltration.

In 1963, Sihanouk accused the United States of supporting antigovernment activities and renounced all U.S. aid. Following a series of border incidents involving South Vietnamese troops, Cambodia in 1965 severed diplomatic relations with the United States. Sihanouk remained on friendly terms with the Communist countries, especially Communist China, and established close relations with France. Economic conditions deteriorated after the renunciation of U.S. aid, and North Vietnamese and Viet Cong troops continued to infiltrate. In the spring of 1969 the United States instituted aerial attacks against Communist strongholds in Cambodia; these bombings, carefully kept secret from the American people, later became an important issue in U.S. politics. As Communist infiltration increased, Sihanouk began to turn more toward the West, and in July, 1969, diplomatic ties with the United States were restored. Relations with South Vietnam and Thailand, after years of border disputes and incidents, began to improve.

In Aug., 1969, Lt. Gen. Lon Nol, the defense minister and supreme commander of the army, became premier, with Sihanouk delegating considerable power to him. Sihanouk began negotiating for the removal of Viet Cong and North Vietnamese troops, who now numbered over 50,000 and occupied large areas of Cambodia. His actions, however, were not enough to ease the growing concern of many army leaders. Discontent with Sihanouk's rule was further heightened by rising inflation, ruinous financial policies, and governmental corruption and mismanagement. On Mar. 18, 1970, while Sihanouk was in Moscow seeking help against further North Vietnamese incursions, premier Lon Nol led a right-wing coup deposing Sihanouk as chief of state. Sihanouk subsequently set up a government-in-exile in Beijing. Soon after the coup, Cambodian troops began engaging Communist forces on Cambodian soil.

In Apr., 1970, U.S. and South Vietnamese troops entered Cambodia to attack Communist bases and supply lines. U.S. ground forces were withdrawn by June 30, but South Vietnamese troops remained, occupying heavily populated areas. The actions of the South Vietnamese troops in Cambodia and the resumption of heavy U.S. air bombings in their support, with the inevitable destruction of villages and killing of civilians, alienated many Cambodians and created considerable sympathy for the Communists. The number of Cambodian Communists (known as the Khmer Rouge) increased from about 3,000 in Mar., 1970, to over 30,000 within a few years. Most of the North Vietnamese and Viet Cong troops were able to withdraw, leaving in progress a raging civil war fought by Cambodians but financed by the United States, North Vietnam, and Communist China.

On Oct. 9, 1970, the national assembly declared Cambodia a republic and changed the country's name to the Khmer Republic. By that time, however, the national government controlled less than one third of Cambodia's total land area: Phnom Penh, most of the provincial capitals, and the central plain S of Tônlé Sap. Despite extensive U.S. military aid, the Khmer Rouge retained firm control of the northeast provinces and most of the countryside. Eventually, more and more territory fell into Communist hands, despite intensive U.S. bombing attacks which persisted until the halt imposed by the U.S. Congress in Aug., 1973.

The government's military position became desperate, with government forces concentrating primarily on keeping communications open with an increasingly beleaguered Phnom Penh. In Sept., 1972, severe food shortages in Phnom Penh sparked two days of rioting and large-scale looting, in which government troops participated. Lon Nol, aided by his brother Lon Non, exerted an increasingly oppressive rule, with massive political arrests and newspaper seizures. The Khmer Rouge insurgents launched a large-scale attack against Cambodia's third largest city, Kompong Cham, in Sept., 1973, and shelled Phnom Penh in 1974 and 1975, inflicting heavy casualties.

In 1975, the Khmer Rouge, led by Pol Pot, seized control of Phnom Penh and overthrew the U.S.-backed government of Lon Nol. The Khmer Rouge renamed the country the Democratic Kampuchea, and established Pol Pot as the premier. Immediately following the takeover, Phnom Penh was evacuated, and the entire population of the country's urban areas was forced to move to rural areas and work in agriculture. Most of the country's vehicles and machines were destroyed because the new regime was opposed to technology and Western influence. It is estimated that about a million and a half people were executed by the Khmer Rouge over the next four years. Members of the upper, middle, or educated classes, as well as suspected enemies of the Khmer Rouge, were victims of the genocide.

In 1978, after Pol Pot refused offers of negotiation and international supervision, the Vietnamese army invaded and seized Phnom Penh in 1979. Prince Sihanouk, who had been imprisoned in his palace by the Khmer Rouge, again fled to Beijing. The Khmer Rouge was driven into the western countryside, but the Kampuchean People's Republic, led by Pol Pot, was still recognized by the United Nations as the country's legitimate government. Throughout the 1980s various guerrilla factions formed and skirmished with the Vietnamese and the Khmer Rouge. One such group was a coalition force led by Sihanouk, who was still recognized by many Cambodians as the country's true leader.

In 1987 talks began in Paris to try to settle the civil war, and in 1989, Vietnam announced plans to withdraw its occupying troops from Cambodia. A peace treaty was signed by all of Cambodia's warring factions (including the Khmer Rouge, Hun Sen's Vietnamese-supported government, and Prince Sihanouk's faction) on Oct. 23, 1991. As agreed in the treaty, the United Nations assumed (1992) the government's administrative functions and worked toward democratic elections. However, provisions calling for disarmament of all factions were resisted by the Khmer Rouge, who resumed guerrilla warfare. Sihanouk denounced the Khmer Rouge, aligned himself with Premier Hun Sen, and again became head of state.

Cambodia's first-ever democratic elections were held in May, 1993, supervised by a large UN peacekeeping mission. Royalists won the largest bloc of national assembly seats (58 out of 120); Hun Sen's party came in second, and a coalition government with co-premiers—Prince Norodom Ranariddh and Hun Sen—was formed. The government administration remained populated largely by bureaucrats who had operated under the Hun Sen regime. The Khmer Rouge, who had boycotted the elections, continued armed opposition, retaining control of substantial territory in the N and W parts of the country. A new constitution reestablished the monarchy, and in Sept., 1993, Sihanouk became king. Attempts at mediation with the Khmer Rouge failed, and fighting continued.

In 1996 the Khmer Rouge split into two factions, one of which made an accord with the government. Pol Pot was ousted and imprisoned by the remaining Khmer Rouge in 1997 and died in 1998; the Khmer Rouge subsequently lost most of its remaining power and support. Following fighting in July, 1997, between the factions of Hun Sen and Prince Ranariddh, Hun Sen's forces declared victory and Ranariddh fled the country; he was replaced as first premier by Ung Huot. Prince Ranariddh returned to Cambodia in Mar., 1998, and became an opposition candidate in the legislative elections held in July. Hun Sen's party (the Cambodian People's party) was the official winner of the disputed election (with 64 seats out of 122), and he became the sole premier. Prince Ranariddh became the president of the national assembly, but Hun Sen further consolidated his control of the country.

Cambodia joined the Association of Southeast Asian Nations (ASEAN) in 1999. Elections in July, 2003, failed to give Hun Sen's Cambodian People's party (CPP) the two-thirds majority needed to govern without a coalition, but the liberal and royalist opposition parties denounced the results, rejected a two-party coalition, formed the Alliance of Democrats, and insisted that the alliance be the cornerstone of a three-party coalition. The deadlock remained unresolved until June, 2004, when Prince Ranariddh's party agreed to a renewed coalition with the CPP. A 186-member cabinet, the seats in which were reportedly sold for large sums in the expectation that they would yield corrupt profits, was formed.

The king abdicated in Oct., 2004, in favor of his son Norodom Sihamoni, despite the fact that the constitution made no provision for abdication. In Feb., 2005, the national assembly lifted opposition leader Sam Rainsy's parliamentary immunity, subjecting him to potential defamation lawsuits from the governing coalition, which he had accused of corruption. He fled Cambodia, and was subsequently convicted of defamation. Other members of his party also were tried and convicted in trials that international human-rights groups said were shams, and subsequently independent human-rights activists were arrested.

A political truce in early 2006, due in part to pressure from international aid donors, resulted in a pardon for Sam Rainsy and others and in Rainsy's return to Cambodia. In Mar., 2006, the constitution was amended so that future governments could be formed with the support of a majority of the members of parliamemt instead of two thirds of the members. Evidence of corruption led the World Bank to suspend funding for three Cambodian development projects in mid-2006. In July, 2006, a tribunal staffed by both Cambodian and international judges was formed to try former Khmer Rouge leaders; the event marked the culmination of nearly nine years of negotiations concerning such trials. The first trial began in 2009 and resulted (2010) in the conviction of the former prison chief. Indictments of other former leaders were subsequently announced; their trials began in 2011. In Oct., 2006, Prince Ranariddh was ousted as leader of the royalist party while he was out of the country. He was subsequently convicted (2007) in absentia of fraud in the sale of the party's headquarters; Ranariddh denounced the conviction as politically motivated; he was pardoned by the king in 2008.

Tensions flared between Cambodia and Thailand in July, 2008, over the Preah Vihear (Phra Viharn) temple on their border. Claimed by both nations but awarded to Cambodia in 1962, it became a UNESCO World Heritage site in 2008. Thai government support for that distinction became a Thai domestic political issue, sparking strong nationalism in both nations and creating a crisis between them. The reinforcement of troops along the border near the temple also led to concern over possible fighting, but in August both sides withdrew most of their forces from the area. Tensions increased in September, however, and there was a brief outbreak of fighting the following month; clashes erupted again in Apr., 2009, and recurred in subsequent years; a demilitarized zone was established around the temple in 2012. Relations with the Thais worsened again in Nov., 2009, when Hun Sen appointed Thaksin Shinawatra, Thailand's deposed prime minister, as an adviser.

Cambodian parliamentary elections in July, 2008, resulted in a landslide for Premier Hun Sen, whose party received nearly 60% of the vote. International observers termed the election flawed but the result largely valid; Cambodian opposition parties, however, denounced the result as manipulated but ultimately accepted the outcome. Government corruption remains a significant problem in Cambodia. In Jan. and Sept., 2010, opposition leader Sam Rainsy was convicted in absentia on charges relating to his actions concerning the marking of the Cambodia-Vietnam border; he had questioned the government's marking of the border, saying it encroached on Cambodian territory, and had removed border markers during a political protest. There was a significant and deadly border clash between Thai and Cambodian forces at the Preah Vihear temple in Feb., 2011, and there and at other sites in April. In Dec., 2011, both sides agreed to withdraw their forces from the disputed areas around Preah Vihear. Cambodia experienced significant flooding during the 2011 monsoon season; by October, three quarters of the country's provinces had been affected by floodwaters.