In early 1958, von Horn was appointed chief of United Nations Truce Supervision Organization in Palestine by Dag Hammarskjöld, and was then promoted to major general. At first he allowed himself "to be optimistic" and he wrote in his memoirs that "instead of fully armed troops, tanks and artillery or even the threat of sanctions, Dag's moral support and my own determination was everything I had." In 1960 he became with short notice the Supreme Commander of the UN Force in the Congo during the beginning of what was to develop into the Congo Crisis, a mission, however, he was dismissed from six months later. von Horn then returned to work for the UN in Palestine. In 1963 he was sent to lead the UN observatory group United Nations Yemen Observation Mission in Yemen where fighting continued between government troops and rebels.