Cambodia ( (listen); also Kampuchea ; Khmer: , Kmpcha [kampuci]), officially the Kingdom of Cambodia, is a country located in the southern portion of the Indochinese Peninsula in Southeast Asia, spanning an area of 181,035 square kilometres (69,898 square miles), 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 Tonl 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 the CambodianVietnamese War. The Vietnamese-occupied People's Republic of Kampuchea became the de facto government, with attempts to rebuild the country after the genocide mired by limited international recognition and ongoing conflict.

Following the 1991 Paris Peace Accords which formally ended the war with Vietnam, Cambodia was governed briefly by a United Nations mission (199293). The UN withdrew after holding elections in which around 90 percent of the registered voters cast ballots. The 1997 coup d'tat consolidated power under Prime Minister Hun Sen and the Cambodian People's Party, who remain in power.

The United Nations designates Cambodia as a least developed country. Cambodia is a member of the United Nations, ASEAN, the RCEP, the East Asia Summit, the WTO, the Non-Aligned Movement and La Francophonie. Although constitutionally a multi-party state, the political system de-facto centres around a single political party, the Cambodian People's Party. While per capita income remains low compared to most neighboring countries, Cambodia has one of the fastest-growing economies in Asia. Agriculture remains the dominant economic sector, with strong growth in textiles, construction, garments, and tourism leading to increased foreign investment and international trade. Rich in biodiversity and seasonal tropical forests, Cambodia has a high rate of deforestation and is considered among the most vulnerable countries to climate change. Foreign observers have highlighted corruption, poverty and human rights as major issues in the country.

The Vietnam War (Vietnamese: Chiến tranh Việt Nam), also known as the Second Indochina War, was a conflict in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. It was the second of the Indochina Wars and was officially fought between North Vietnam and South Vietnam. North Vietnam was supported by the Soviet Union, China, and other communist allies; South Vietnam was supported by the United States and other anti-communist allies. The war is widely considered to be a Cold War-era proxy war. It lasted almost 20 years, with direct U.S. involvement ending in 1973. The conflict also spilled over into neighboring states, exacerbating the Laotian Civil War and the Cambodian Civil War, which ended with all three countries becoming communist states by 1975.

The conflict emerged from the First Indochina War between the French colonial government and a left-wing revolutionary movement, the Viet Minh. After the French military withdrawal from Indochina in 1954, the U.S. assumed financial and military support for the South Vietnamese state. The Việt Cộng (VC), a South Vietnamese common front under the direction of North Vietnam, initiated a guerrilla war in the south. North Vietnam had also invaded Laos in 1958 in support of insurgents, establishing the Ho Chi Minh Trail to supply and reinforce the Việt Cộng.: 16  By 1963, the North Vietnamese had sent 40,000 soldiers to fight in the south.: 16  U.S. involvement escalated under President John F. Kennedy through the MAAG program, from just under a thousand military advisors in 1959 to 23,000 in 1964.: 131 In the Gulf of Tonkin incident in August 1964, a U.S. destroyer clashed with North Vietnamese fast attack craft. In response, the U.S. Congress passed the Gulf of Tonkin Resolution and gave President Lyndon B. Johnson broad authority to increase U.S. military presence in Vietnam. Johnson ordered the deployment of combat units for the first time and increased troop levels to 184,000. The People's Army of Vietnam (PAVN) (also known as the North Vietnamese Army or NVA) engaged in more conventional warfare with U.S. and South Vietnamese forces (Army of the Republic of Vietnam (ARVN)). Despite little progress, the U.S. continued a significant build-up of forces. U.S. and South Vietnam forces relied on air superiority and overwhelming firepower to conduct search and destroy operations, involving ground forces, artillery, and airstrikes. The U.S. also conducted a large-scale strategic bombing campaign against North Vietnam.: 371–4 The communist Tet Offensive throughout 1968 caused U.S. domestic support for the war to fade. The VC sustained heavy losses during the Offensive and subsequent U.S.-ARVN operations.: 481  The CIA's Phoenix Program further degraded the VC's membership and capabilities. By the end of the year, the VC insurgents held almost no territory in South Vietnam, and their recruitment dropped by over 80%, signifying a drastic reduction in guerrilla operations, necessitating increased use of PAVN regular soldiers from the north. In 1969, North Vietnam declared a Provisional Revolutionary Government (the PRG) in the south to give the reduced VC a more international stature, but from then on, they were sidelined as PAVN forces began more conventional combined arms warfare. By 1970, over 70% of communist troops in the south were northerners, and southern-dominated VC units no longer existed. Operations crossed national borders: North Vietnam used Laos as a supply route early on, while Cambodia was also used starting in 1967; the U.S. bombed the Laotian route starting in 1964, and the Cambodian route in 1969. The deposing of the monarch Norodom Sihanouk by the Cambodian National Assembly resulted in a PAVN invasion of the country at the request of the Khmer Rouge, escalating the Cambodian Civil War and resulting in a U.S.-ARVN counter-invasion.

In 1969, following the election of U.S. President Richard Nixon, a policy of "Vietnamization" began, which saw the conflict fought by an expanded ARVN, with U.S. forces sidelined and increasingly demoralized by domestic opposition and reduced recruitment. U.S. ground forces had largely withdrawn by early 1972 and support was limited to air support, artillery support, advisers, and materiel shipments. The ARVN, with U.S. support, stopped the first and largest mechanized PAVN offensive during the Easter Offensive of 1972. The offensive failed to subdue South Vietnam, but the ARVN itself failed to recapture all lost territory, leaving its military situation difficult. The Paris Peace Accords of January 1973 saw all U.S. forces withdrawn; the Case–Church Amendment, passed by the U.S. Congress on 15 August 1973, officially ended direct U.S. military involvement.: 457  The Peace Accords were broken almost immediately, and fighting continued for two more years. Phnom Penh fell to the Khmer Rouge on 17 April 1975, while the 1975 Spring Offensive saw the Fall of Saigon by the PAVN on 30 April; this marked the end of the war, and North and South Vietnam were reunified the following year.

By 1970, the ARVN was the world's fourth largest army, and the PAVN was not far behind with approximately one million regular soldiers.: 770  The war exacted an enormous human cost: estimates of the number of Vietnamese soldiers and civilians killed range from 966,000 to 3 million. Some 275,000–310,000 Cambodians, 20,000–62,000 Laotians, and 58,220 U.S. service members also died in the conflict, and a further 1,626 remain missing in action.The Sino-Soviet split re-emerged following the lull during the Vietnam War. Conflict between North Vietnam and its Cambodian allies in the Royal Government of the National Union of Kampuchea, and the newly formed Democratic Kampuchea began almost immediately in a series of border raids by the Khmer Rouge, eventually escalating into the Cambodian–Vietnamese War. Chinese forces directly invaded Vietnam in the Sino-Vietnamese War, with subsequent border conflicts lasting until 1991. The unified Vietnam fought insurgencies in all three countries. The end of the war and resumption of the Third Indochina War would precipitate the Vietnamese boat people and the larger Indochina refugee crisis, which saw millions of refugees leave Indochina (mainly southern Vietnam), an estimated 250,000 of whom perished at sea. Within the U.S., the war gave rise to what was referred to as Vietnam Syndrome, a public aversion to American overseas military involvements, which together with the Watergate scandal contributed to the crisis of confidence that affected America throughout the 1970s.