Cambodia's history is characterized by periods of peace and tragedy. Archaeological findings indicate human presence and rice cultivation as early as 4000 BC. Early Cambodians likely migrated from the north, and by the 1st century AD, Chinese traders reported on sophisticated coastal kingdoms influenced by Indian culture in their arts, alphabets, religions (Buddhism and Hinduism), architectural styles, and caste systems. The ancient state of Funan, from the 1st to 6th centuries, was the earliest Indianized state in the region, located in the lower Mekong and known for its prolonged interactions with maritime traders, leading to the evolution of the Cambodian language with Sanskrit elements.

Funan gave way to the Angkor Empire, founded by King Jayavarman II in 802 AD. This period marked 600 years of political and powerful dominance by Khmer kings across Southeast Asia, from Myanmar to the South China Sea and Laos. These kings were renowned for constructing the Angkor temple complex, the world's most extensive concentration of religious temples. Angkorian kings, such as Jayavarman II, Indravarman I, Suryavarman II, and Jayavarman VII, made significant advancements in agriculture with sophisticated irrigation systems, creating massive artificial lakes and canals that allowed for three rice crops annually. They also excelled in architectural developments, urban planning, and logistics, with some structures still in use today.

From the 9th to the 15th century, over 26 kings ruled Cambodia, building more than a thousand temples, including Angkor Wat by Suryavarman II and the Bayon temple by Jayavarman VII. Jayavarman VII, a staunch Buddhist, also established hospitals and inns. Despite its peak in the 12th century, covering areas of Vietnam, Laos, Thailand, and Myanmar, the Khmer kingdom faced continuous internal rebellions and foreign invasions. Factors contributing to its 13th and 14th-century decline included the rise of powerful Thai empires, population drop from continuous wars, the introduction of Theravada Buddhism challenging the monarchical society, extended droughts due to climate change, and increased maritime activities that weakened Angkor's stronghold. Following Thai invasions, the Cambodian elite moved to Phnom Penh.

From the 15th to the 19th century, Cambodia experienced a 'dark age' of deterioration and territorial loss. A brief respite in the 16th century saw renewed trade, but the Siamese overthrow of the capital Long Vec in 1594 led to Cambodia becoming a puppet state between Siam and Vietnam. In the 17th century, Vietnam's settlement of the Mekong Delta resulted in Cambodia losing wealthy territories and access to the sea. This foreign encroachment continued into the 19th century, with Vietnam attempting to absorb Khmer lands and impose its culture. In 1863, France established a protectorate over Cambodia, saving it from further disintegration by Thailand and Vietnam. The French maintained an indirect rule, preserving much of Cambodia's institutions and monarchy, while developing civil services, roads, port facilities, and rubber plantations.

During World War II, the Japanese entered Cambodia but left the French administration intact until 1945, when they installed an independent Cambodian king, Norodom Sihanouk. After the Japanese departure in 1946, the French returned, allowing Cambodians to draft a constitution. The First Indochina War (1946-1954) saw communist nationalist groups fight for independence. King Sihanouk negotiated Cambodia's independence from the French in 1953 without bloodshed, recognized by the Geneva Accord of 1954. In 1955, Sihanouk abdicated to pursue political ambitions, forming the Sangkum Reastr Niyum party, winning all seats in the national election, and later becoming head of state. During the Cold War, Sihanouk maintained neutrality, leading to economic expansion through aid from the US, China, and the USSR.

By 1965, Sihanouk broke ties with the US, allowing North and South Vietnamese forces to establish bases on Cambodian soil, intensifying local tensions. The Cambodian Communist Party of Kampuchea (CPK or Khmer Rouge) took up arms against the government. Facing an unstable economy, Sihanouk renewed ties with the US, leading to President Nixon ordering the bombing of Cambodia in 1969 to destroy Vietnamese sanctuaries. In March 1970, Sihanouk was removed by the national assembly, and General Lon Nol assumed power, forming the Khmer Republic in October 1970. Sihanouk, in exile, allied with North Vietnam and the Khmer Rouge. The US stopped bombing in 1973, leaving Lon Nol's forces to fight the Vietnamese and Khmer Rouge amidst widespread destruction. In 1975, the Khmer Republic fell, and the Khmer Rouge took Phnom Penh.

Under the command of Pol Pot (Saloth Sar), the Khmer Rouge regime, known as Democratic Kampuchea (DK), forced all city dwellers into the countryside for farming. DK advocated for total independence but accepted support from China and North Korea. The regime introduced harsh socialist policies, restricting freedom of speech, association, movement, and banning religious practices, while also regulating food and information. The regime was particularly brutal towards former city dwellers and anyone opposing them, including religious leaders, bureaucrats, merchants, and ordinary citizens. Millions were forcibly relocated, starved, subjected to forced labor or torture. The Khmer Rouge was responsible for the deaths of nearly 1.7 million Cambodians, about one-fifth of the population, and also attacked neighboring countries and massacred ethnic Vietnamese and Chinese minorities.

The Khmer Rouge regime ended in 1979. In October 1991, all warring factions, along with the UN and other foreign nations, agreed to end the conflict through an accord that established a power-sharing arrangement between the UN Transitional Authority in Cambodia and the Supreme National Council, led by Prince Norodom Sihanouk. In 1993, a new constitution restored the monarchy, and Cambodia's kingdom was re-established, with Sihanouk becoming king again after elections. The DK had lost international support and its UN seat. Today, Cambodia's economy relies heavily on foreign aid, and internal political crises continue to challenge its democratization and economic growth.