From Katanga 1960-63: Mercenaries, Spies and the African Nation that Waged War on the World, by Christopher Othen (History Press, 2015), Kindle Loc. ~1026:
Ralph Bunce had passed on Lumumba’s request for help to the United Nations Secretary General, a Swedish civil servant with blonde hair and grey-blue eyes calm as a frozen lake. Dag Hammarskjöld turned it down. The UN’s job was peace.
The United Nations had been around since the end of the Second World War. Its optimistic goal of world harmony was often compromised by the competing desires of America and the Soviet Union, its strongest members. American pressure sent UN troops to the Korean War in 1950 and Soviet demands made them sit and watch as the Red Army crushed anti-communist rebels in Hungary six years later. Most of Hammarskjöld’s energy went into persuading the superpowers occasionally to vote the same way.
The Swede did not want the UN to be used as a private army to take back Katanga. The Congo’s biggest problem, in his view, was the threat of a clash between Belgian soldiers and the ANC. He twisted some superpower arms and secured a mandate from the Security Council in New York to replace the 7,400 Belgians in the Congo with UN soldiers. The first peacekeepers, a Tunisian contingent, arrived in Léopoldville on 14 July, followed by units from Ghana, Mali and Morocco. Belgian soldiers reluctantly gave up their positions to blue-helmeted UN men and flew home. The process was surprisingly smooth, even surviving a kick in the teeth from Lumumba, when he declared it too slow and asked the Soviet Union to intervene independently. Moscow officially declined but saw a chance to sink its claws into Africa. Soviet aeroplanes and lorries and Czechoslovak technicians began to arrive secretly in Stanleyville. Cold warriors in Brussels were horrified.
‘The Congo will become communist within two months,’ said Harold d’Aspremont-Lynden, a close colleague of the Belgian prime minister.
Soon after, Harold d’Aspremont-Lynden was on his way to Katanga as head of the Mission Technique Belge (Belgian Technical Mission – Mistebel), a high-powered group of experts full of ideas on how to run the new country. Minister of Foreign Affairs Pierre Wigny was not happy. He had been arguing against taking sides in Katanga ever since Tshombe declared independence, but lost any support in the Cabinet after Léopoldville accused Brussels of organising the secession and broke diplomatic relations.