The Kingdom of Cambodia, also known as the First Kingdom of Cambodia, and commonly referred to as the Sangkum period, refers to Norodom Sihanouk's first administration of Cambodia, lasting from the country's independence from France in 1953 to a military coup d'état in 1970. Sihanouk continues to be one of the most controversial figures in Southeast Asia's turbulent and often tragic postwar history. From 1955 until 1970, Sihanouk's Sangkum was the sole legal party in Cambodia. After the 1970 coup, the semi-official name of Cambodia was State of Cambodia, it became a transitional government under a military dictatorship of Prime Minister Lon Nol against the Khmer Rouge and North Vietnamese.

Following the end of World War II, France restored its colonial control over Indochina but faced local resistance against their rule, particularly from Communist guerilla forces. On 9 November 1953, Cambodia achieved independence from France under Norodom Sihanouk but still faced resistance from Communist groups such as United Issarak Front. As the Vietnam War escalated, Cambodia sought to retain its neutrality but in 1965, North Vietnamese soldiers were allowed to set up bases and in 1969, the United States began a bombing campaign against North Vietnamese soldiers in Cambodia. The Cambodian monarchy was abolished in a coup on October 9, 1970 headed by Prime Minister Lon Nol, who established the Khmer Republic which lasted until the Fall of Phnom Penh to the Khmer Rouge led by Angkar ("Organization") when it came to power on 17 April 1975.

Although Cambodia had achieved independence by late 1953, its military situation remained unsettled. Noncommunist factions of the Khmer Issarak had joined the government, but pro-communist Viet Minh and United Issarak Front activities increased at the very time French Union forces were stretched thin elsewhere. In April 1954, several Viet Minh battalions crossed the border into Cambodia. Royalist forces engaged them but could not force their complete withdrawal. In part, the communists were attempting to strengthen their bargaining position at the Geneva Conference that had been scheduled to begin in late April.

The Geneva Conference was attended by representatives of Cambodia, North Vietnam, the Associated State of Vietnam (the predecessor of the Republic of Vietnam or South Vietnam), Kingdom of Laos, the People's Republic of China, the Soviet Union, Britain, France and the United States. One goal of the conference was to restore a lasting peace in Indochina. The discussions on Indochina began on May 8, 1954. The North Vietnamese attempted to get representation for the resistance government that had been established in the south, but failed.

On July 21, 1954, the conference reached an agreement calling for a cessation of hostilities in Indochina. With respect to Cambodia, the agreement stipulated that all Viet Minh military forces be withdrawn within ninety days and that Cambodian resistance forces be demobilized within thirty days. In a separate agreement signed by the Cambodian representative, the French and the Viet Minh agreed to withdraw all forces from Cambodian soil by October 1954.

In exchange for the withdrawal of Viet Minh forces, the communist representatives in Geneva wanted full neutrality for Cambodia and for Laos that would prevent the basing of the United States Armed Forces in these countries. On the eve of the conference's conclusion, however, the Cambodian representative, Sam Sary, insisted that, if Cambodia were to be genuinely independent, it must not be prohibited from seeking whatever military assistance it desired (Cambodia had earlier appealed to the U.S. for military aid). The conference accepted this point over North Vietnam's strenuous objections. In the final agreement, Cambodia accepted a watered-down neutrality, vowing not to join any military alliance "not in conformity with the principles of the Charter of the United Nations" or to allow the basing of foreign military forces on its territory "as long as its security is not threatened."

The conference agreement established the International Control Commission (officially called the International Commission for Supervision and Control) in all the Indochinese countries. Made up of representatives from Canada, India and Poland, it supervised the cease-fire, the withdrawal of foreign troops, the release of prisoners of war and overall compliance with the terms of the agreement. The French and most of the Viet Minh forces were withdrawn on schedule in October 1954.

The Geneva agreement also stipulated that general elections should be held in Cambodia during 1955 and that the International Control Commission should monitor them to ensure fairness. Sihanouk was more determined than ever to defeat the Democrats (who, on the basis of their past record, were expected to win the election). The king attempted unsuccessfully to have the constitution amended. On March 2, 1955, he announced his abdication in favor of his father, Norodom Suramarit. Assuming the title of Samdech (meaning "Lord" but in this context "Prince"), Sihanouk explained that this action was necessary in order to give him a free hand to engage in politics.