In 1935 Hammarskjöld became Sweden's undersecretary of finance and in 1945 was named economic advisor to the prime minister's cabinet. Hammarskjöld joined the Swedish foreign ministry in 1949 and was appointed deputy foreign minister in 1951, choosing to remain aloof from domestic political affairs. He also served for some years as chairman of the National Bank's board, was a member of numerous delegations to international conferences, and served as acting chairman of Sweden's delegation to the seventh General Assembly Conference in 1952–1953.
Hammarskjöld was elected UN secretary-general in April 1953 as a dark-horse candidate known for his technical skills rather than his political prowess. He was unanimously reelected in September 1957. He spent his first years concentrating on strengthening and streamlining the UN's administrative staff and cultivating confidence among UN members. As such, he launched his concept of quiet diplomacy as a complement to the General Assembly's parliamentary diplomacy. His 1955 Beijing mission, which led to the release of fifteen U.S. airmen imprisoned for espionage, demonstrated the inherent efficacy of his quiet diplomacy approach to international issues. His role in defusing the 1956 Suez Crisis helped contribute to the growing prestige and authority of the UN.
During his second term, Hammarskjöld developed an even more active political profile, aimed at preventive measures to deter war and international tensions. As he saw it, one of the secretary-general's tasks was to promote Cold War rapprochement by mitigating outstanding Cold War issues and potential flash points. This implied the safeguarding of newly independent states to prevent them from being drawn into the superpower rivalry. Applying his ideas to the Congo, he came into conflict with Soviet leader Nikita Khrushchev, who in September 1960 demanded Hammarskjöld's replacement.
Hammarskjöld remained in office but met with increasing difficulties as he tried to mediate conflicts in the newly independent Congo and as he fought off criticism from some UN members, most notably the Soviet Union. He was killed on 18 September 1961 in a plane crash near the Katanga-North Rhodesia border while on a peace mission to the Congo. In late 1961, Hammarskjöld was the first person to receive the Nobel Peace Prize posthumously. His spiritual journal Markings, first published in Swedish in 1963, bears witness to his upstanding character and the centrality of his Christian faith to his life's work.
Hammarskjöld, Dag. Markings. New York: Knopf, 1964.; Heller, Peter B. The United Nations under Dag Hammarskjold, 1953–1961. Lanham, MD: Scarecrow, 2001.; Urquhart, Brian. Hammarskjold. New York: Knopf, 1972.