Kasavubu staged an army coup in 1960 and handed Lumumba over to the Katangan forces. A UN investigating commission found that Lumumba had been killed by a Belgian mercenary in the presence of Tshombe, who was then the president of Katanga. U.S. and Belgian involvement in the assassination have been alleged. In a possibly related development, Dag Hammarskjold, UN secretary-general, died in a plane crash en route to a peace conference with Tshombe on Sept. 17, 1961.

Tshombe rejected a national reconciliation plan submitted by the UN in 1962. Tshombes troops fired on the UN force in December, and in the ensuing conflict Tshombe capitulated on Jan. 14, 1963. The peacekeeping force withdrew, and, in a complete about-face, Kasavubu named Tshombe premier in order to fight a spreading rebellion. Tshombe used foreign mercenaries, and, with the help of Belgian paratroops airlifted by U.S. planes, defeated the most serious opposition, a Communist-backed regime in the northeast.