Moïse Tshombe declared the mineral-rich province of Katanga independent, creating a major challenge to Congo’s territorial integrity and becoming the central political-military issue that ONUC would confront.
The UN Security Council adopted Resolution 143, calling on Belgium to withdraw its troops and authorizing the Secretary-General to provide military assistance to the Congolese government—formalizing the start of ONUC.
The first UN contingents arrived (within days of Resolution 143), enabling Belgium’s initial withdrawals from Léopoldville and beginning ONUC’s rapid force buildup.
The Security Council adopted Resolution 145, urging Belgium to implement troop withdrawals and authorizing the Secretary-General to take necessary action, while calling on states not to undermine Congo’s territorial integrity.
With Resolution 146, the Security Council called for Belgian withdrawal from Katanga and affirmed the need for UN forces to enter Katanga for implementation, while reiterating ONUC would not be a party to internal conflict.
Albert Kalonji proclaimed the independence of South Kasai, further fragmenting the new state and complicating ONUC’s stabilization and territorial-integrity objectives.
Following the killing of Patrice Lumumba (and colleagues), Resolution 161 urged the UN to prevent civil war, including use of force as a last resort, and demanded withdrawal of foreign military/paramilitary personnel and mercenaries not under UN command.
The UN General Assembly adopted A/RES/1595 (XV) on Congo operations’ 1961 costs and financing, reflecting the scale of ONUC and the contentious politics of funding a large armed peacekeeping mission.
ONUC undertook Operation Morthor to curb Katangese forces and foreign mercenaries; the fighting intensified international controversy over ONUC’s use of force and command decisions.
UN Secretary-General Dag Hammarskjöld died when his chartered DC-6 crashed near Ndola while traveling to negotiations related to the Katanga conflict—an event that deeply affected UN leadership and crisis diplomacy.
With Resolution 169, the Security Council condemned secessionist activity in Katanga and authorized the Secretary-General to take necessary action to apprehend and deport foreign military personnel, paramilitaries, and mercenaries not serving under the UN.
ONUC launched Operation Grandslam, a major offensive that broke Katangese armed resistance and led to the reintegration of Katanga, marking the decisive military turning point of the mission.
After ONUC’s military victories, Katanga’s secession was formally ended, allowing ONUC to begin planning a phased drawdown as the most acute external-mercenary-backed secessionist challenge receded.
ONUC completed its withdrawal, ending the UN’s large-scale armed peacekeeping operation in the Congo (though UN civilian assistance continued beyond the mission’s military phase).
United Nations Operation in the Congo (ONUC) (1960–1964)
