Republic of the Congo (Léopoldville) in the context of "Democratic Republic of the Congo"

The Congo Crisis (French: Crise congolaise) was a period of political upheaval and conflict between 1960 and 1965 in the Republic of the Congo (today the Democratic Republic of the Congo). The crisis began almost immediately after the Congo became independent from Belgium and ended, unofficially, with the entire country under the rule of Joseph-Désiré Mobutu. Constituting a series of civil wars, the Congo Crisis was also a proxy conflict in the Cold War, in which the Soviet Union and the United States supported opposing factions. Around 100,000 people are believed to have been killed during the crisis.

A nationalist movement in the Belgian Congo demanded the end of colonial rule: this led to the country's independence on 30 June 1960. Minimal preparations had been made and many issues, such as federalism, tribalism, and ethnic nationalism, remained unresolved. In the first week of July, 1960, a mutiny broke out in the army and violence erupted between black and white civilians. Belgium sent troops to protect fleeing white citizens. Katanga and South Kasai seceded with Belgian support. Amid continuing unrest and violence, the United Nations deployed peacekeepers, but UN secretary-general Dag Hammarskjöld refused to use these troops to help the central government in Léopoldville (Kinshasa) fight the secessionists. Prime Minister Patrice Lumumba, the charismatic leader of the largest nationalist faction, reacted by calling for assistance from the Soviet Union, which promptly sent military advisers and other support.

Belgium controlled several territories and concessions during the colonial era, principally the Belgian Congo (modern DR Congo) from 1908 to 1960, Ruanda-Urundi (modern Rwanda and Burundi) from 1922 to 1962, and Lado Enclave (modern Central Equatoria province in South Sudan) from 1894 to 1910. It also had small concessions in Guatemala (1843–1854) and Belgian concession of Tianjin in China (1902–1931) and was a co-administrator of the Tangier International Zone in Morocco.

Roughly 98% of Belgium's overseas territory was just one colony (about 76 times larger than Belgium itself) – known as the Belgian Congo. The colony was founded in 1908 following the transfer of sovereignty from the Congo Free State, which was the personal property of Belgium's king, Leopold II. The violence used by Free State officials against indigenous Congolese and the ruthless system of economic extraction had led to intense diplomatic pressure on Belgium to take official control of the country. Belgian rule in the Congo was based on the "colonial trinity" (trinité coloniale) of state, missionary and private company interests. During the 1940s and 1950s, the Congo experienced extensive urbanization and the administration aimed to make it into a "model colony". As the result of a widespread and increasingly radical pro-independence movement, the Congo achieved independence, as the Republic of Congo-Léopoldville in 1960.

The United Nations Operation in the Congo (French: Opération des Nations Unies au Congo, abbreviated ONUC) was a United Nations peacekeeping force which was deployed in the Republic of the Congo in 1960 in response to the Congo Crisis. The ONUC was the UN's first peacekeeping mission with significant military capability, and remains one of the largest UN operations in size and scope.

The Congo descended into chaos and disorder after it became independent from Belgium on 30 June 1960. This prompted a swift return of Belgian troops, under the pretext of restoring order and protecting its nationals. In response to the Congolese government's appeal for assistance, on 14 July 1960 the United Nations Security Council passed Resolution 143 (S/4387), which called on Belgium to withdraw its troops and authorized the UN Secretary-General to provide the Congolese government with military assistance. The first UN troops, drawn mostly from African and Asian states, reached the Congo the following day.

On 5 July 1960, soldiers of the garrisons of Léopoldville and Thysville of the Force Publique, the army of the newly independent Republic of the Congo, mutinied against their white officers. The revolt quickly spread throughout the Bas-Congo and engulfed the country in disorder, beginning the Congo Crisis.

South Kasai (French: Sud-Kasaï) was an unrecognised secessionist state within the Republic of the Congo (the modern-day Democratic Republic of the Congo) which was semi-independent between 1960 and 1962. Initially proposed as only a province, South Kasai sought full autonomy in similar circumstances to the much larger neighbouring state of Katanga, to its south, during the political turmoil arising from the independence of the Belgian Congo known as the Congo Crisis. Unlike Katanga, however, South Kasai did not explicitly declare full independence from the Republic of the Congo or reject Congolese sovereignty.

The South Kasaian leader and main advocate, Albert Kalonji, who had represented a faction of the nationalist movement (the Mouvement National Congolais-Kalonji or MNC-K) before decolonisation, exploited ethnic tensions between his own ethnic group, the Baluba, and the Bena Lulua to create a Luba-focused state in the group's traditional heartland in the south-eastern parts of the Kasai region. As sectarian violence broke out across the country, the state declared its secession from the Congo on 9 August 1960 and its government and called for the Baluba living in the rest of the Congo to return to their "homeland". Kalonji was appointed president. Although the South Kasaian government claimed to form an autonomous part of a federal Congo-wide state, it exercised a degree of regional autonomy and even produced its own constitution and postage stamps. The state, supported by foreign powers, particularly Belgium, and funded by diamond exports, managed numerous crises, including those caused by the large emigration of Luba refugees, but became increasingly militarist and repressive.