Dag Hjalmar Agne Carl Hammarskjöld (29 July 1905 – 18 September 1961) was a Swedish diplomat and author and was the second Secretary-General of the United Nations. He served from April 1953 until his death in a plane crash in September 1961. He is the only person to have been awarded the Nobel Peace Prize posthumously. Hammarskjöld remains the only U.N. Secretary-General to die in office. U.S. President John F. Kennedy called Hammarskjöld “the greatest statesman of our century.”

Dag Hammarskjöld was born in Jönköping, Sweden, but he lived most of his childhood in Uppsala. He was the fourth and youngest son of Hjalmar Hammarskjöld, prime minister of Sweden from 1914 to 1917, and Agnes Hammarskjöld (née Almquist). His ancestors had served the Swedish Crown since the 17th century. He studied first at Katedralskolan and then at Uppsala University where he graduated with a Master's degree in Political Economy and a Bachelor of Law degree. He then moved to Stockholm.

From 1930 to 1934, he was a secretary of a governmental committee on unemployment. He also wrote his economics thesis, Konjunkturspridningen (The Spread of the Business Cycle), and received his doctorate from Stockholm University in 1933. In 1936, Hammarskjöld became a secretary at the Bank of Sweden, and soon he was an undersecretary of finance. From 1941 to 1948, he served as chairman of the Bank of Sweden. Early in 1945, he was appointed as adviser to the cabinet on financial and economic problems, and he coordinated government plans to alleviate the economic problems of the post-war period.

In 1947, Hammarskjöld was appointed to Sweden’s Ministry for Foreign Affairs, and in 1949 he became the state secretary for foreign affairs. He was a delegate to the Paris conference that established the Marshall Plan. In 1948, he was again in Paris to attend a conference for the Organisation for European Economic Co-operation. In 1950, he became head of the Swedish delegation to UNISCAN. In 1951, he became a cabinet minister without portfolio and in effect deputy foreign minister. Although Hammarskjöld served in a cabinet dominated by the Social Democrats, he never officially joined any political party. In 1951, Hammarskjöld became vice chairman of the Swedish delegation to the United Nations General Assembly in Paris. He became the chairman of the Swedish delegation to the General Assembly in New York in 1952. On 20 December 1954, he was elected to take his father's vacated seat in the Swedish Academy.

When Trygve Lie resigned from his post as UN Secretary-General in 1953, the Security Council decided to recommend Hammarskjöld for the post. It came as a surprise to him. He was selected on 31 March by a majority of 10 out of eleven states. The UN General Assembly elected him in the 7–10 April session, by 57 votes out of 60. In 1957, he was re-elected. Hammarskjöld began his term by establishing his own secretariat of 4,000 administrators. He set up regulations that defined their responsibilities. He was also actively engaged in smaller projects relating to the UN working environment; for example, he planned and supervised in every detail the creation of a "meditation room" in the UN headquarters, a place dedicated to silence, where people could withdraw into themselves, regardless of their faith, creed or religion.

During his term, Hammarskjöld tried to smooth relations between Israel and the Arab states. In 1955, he went to China to negotiate the release of 15 US pilots who had served in the Korean War and had been captured by the Chinese. In 1956, following a proposal by future Canadian Prime Minister Lester B. Pearson, the United Nations Emergency Force (UNEF) was established, which allowed the Secretary-General to take emergency action without the prior approval of either the Security Council or General Assembly. In 1957, Hammarskjöld intervened in the Suez Crisis. He is given credit by some historians for allowing the participation of the Holy See within the United Nations that year. He was nicknamed the secular pope by some authors.

In 1960, the former Belgian colony and then newly independent Congo asked for UN aid in defusing escalating civil strife. Hammarskjöld made four trips to the Congo. His efforts towards the decolonisation of Africa were considered insufficient by the Soviet Union; in September 1960, they denounced his decision to send a UN emergency force to keep the peace. They demanded his resignation and the replacement of the office of Secretary-General by a three-man directorate with a built-in veto, the "troika". The objective was, citing the memoirs of the Soviet leader, Nikita Khrushchev, to “equally represent interests of three groups of countries: capitalist, socialist and recently independent.” Hammarskjöld denied Patrice Lumumba's request to help force the Katanga Province to rejoin the Congo, causing Lumumba to turn to the Soviets for help. He personally disliked Lumumba and felt that he should be removed from office.

In September 1961, Hammarskjöld found out about the fighting between non-combatant UN forces and Katangese troops of Moise Tshombe. He was en route to negotiate a cease-fire on the night of 17–18 September when his DC-6B airliner (SE-BDY) crashed near Ndola, Northern Rhodesia (now Zambia). The crew had filed no flight plan, for security reasons, and a decoy aircraft (OO-RIC) went via a different route ahead of Hammarskjöld's aircraft. Hammarskjöld and fifteen others perished in the crash. The chief of security on the flight, American Sgt. Harold Julian, was thrown clear of the burnt area, but died five days later. A memorial was created at the crash site, which is under consideration for inscription as a UNESCO World Heritage Site.

A special report issued by the United Nations following the crash stated that the United Nations base operations at the Ndjili Airport reported that an unidentified aircraft had been overflying the Ndola Airport late the previous night, but that no communication was made. The message also indicated that a report had reached the police station to the effect of a bright flash in the sky at approximately 1 am the previous night. According to the UN special report, it was this information that resulted in the initiation of search and rescue operations. A press release issued by the Prime Minister of the Republic of the Congo, attached to the UN report, stated that "... in order to pay a tribute to this great man [Hammarskjöld], now vanished from the scene, and to his colleagues, all of whom have fallen victim to the shameless intrigues of the great financial Powers of the West, and in order to demonstrate publicly our indignation at the scandalous interference in our affairs by certain foreign countries, the Government has decided to proclaim Tuesday, 19 September 1961, a day of national mourning." These initial indications that the crash may have been deliberate led to multiple official inquiries and persistent speculation that the Secretary-General was assassinated.

Following the death of Hammarskjöld, there were three inquiries into the circumstances that led to the crash: the Rhodesian Board of Investigation, the Rhodesian Commission of Inquiry, and the United Nations Commission of Investigation. The Rhodesian Board of Investigation looked into the matter between 19 September 1961 and 2 November 1961 under the command of British Lt. Colonel M.C.B. Barber. The Rhodesian Commission of Inquiry held hearings from 16–29 January 1962 without United Nations oversight. The subsequent United Nations Commission of Investigation held a series of hearings in 1962 and in part depended upon the testimony from the previous Rhodesian inquiries. Five "eminent persons" were assigned by the new Secretary-General to the UN Commission. The members of the commission unanimously elected Nepalese diplomat Rishikesh Shaha to head up an inquiry. The three official inquiries failed to conclusively determine the cause of the crash that led to the death of Hammarskjöld. The Rhodesian Board of Investigation sent 180 men to search a six square kilometer area of the last sector of the aircraft's flight-path, looking for evidence as to the cause of the crash. No evidence of a bomb, surface-to-air missile, or hijacking was found. The official report stated that two of the dead Swedish bodyguards had suffered multiple bullet wounds. Medical examination, performed by the initial Rhodesian Board of Investigation and reported in the UN official report, indicated that the wounds were superficial, and that the bullets showed no signs of rifling. They concluded that the bullets exploded in the fire in close proximity to the bodyguards. No other evidence of foul play was found in the wreckage of the aircraft. Previous accounts of a bright flash in the sky were dismissed as occurring too late in the evening to have caused the crash. The official UN report speculated that these flashes may have been caused by secondary explosions after the crash. The sole survivor, Sergeant Harold Julian, indicated that there was a series of explosions that preceded the crash. The official inquiry found, however, that the statements of witnesses who talked with Julian were inconsistent. It was concluded that this testimony could not establish that the explosions did not occur after the crash. The report does state that there were numerous delays which violated the established search and rescue procedures. There were three separate delays: the first delayed the initial alarm of a possible plane in trouble; the second delayed the "distress" alarm, which indicates that communications with surrounding airports indicate that a missing plane has not landed elsewhere; the third delayed the eventual search and rescue operation and the discovery of the plane wreckage, just miles away. The medical examiners report was inconclusive; one report said that Hammarskjöld had died on impact; another stated that Hammarskjöld might have survived had rescue operations not been delayed. The report also said that the chances of Sgt. Julian surviving the crash would have been "infinitely" better if the rescue operations were hastened.

Despite the multiple official inquiries that failed to find evidence of assassination, some continue to believe that the death of Hammarskjöld was not an accident. Harry Truman is reported to have said that "Dag Hammarskjöld was on the point of getting something done when they killed him. Notice that I said, 'when they killed him'." At the time of Hammarskjöld's death, western intelligence agencies were actively involved in the political situation in the Congo, which culminated in Belgian support for the secession of Katanga and the assassination of former prime minister Patrice Lumumba. Belgium and the United Kingdom had a vested interest in maintaining their control over much of the country's copper industry during the Congolese transition to an independent state. Concerns about the nationalization of the copper industry could have provided a financial incentive to remove either Lumumba or Hammarskjöld. Belgium has since publicly acknowledged and apologized for its negligence in the death of Lumumba.

The involvement of British officers in commanding the initial inquiries, which provided much of the information about the condition of the plane and the examination of the bodies, have led some to suggest a conflict of interest. The official report dismissed a number of pieces of evidence that would have supported the view that Hammarskjöld was assassinated. Some of these dismissals have been controversial, such as the conclusion that bullet wounds could have been caused by bullets exploding in a fire. Expert tests have questioned this conclusion, arguing that exploding bullets could not break the surface of the skin. Major C. F. Westell, a ballistics authority, said, "I can certainly describe as sheer nonsense the statement that cartridges of machine guns or pistols detonated in a fire can penetrate a human body." He based his statement on a large scale experiment that had been done to determine if military fire brigades would be in danger working near munitions depots. Other Swedish experts conducted and filmed tests showing that bullets heated to the point of explosion nonetheless did not achieve sufficient velocity to penetrate their box container.

Although there is some skepticism as to whether the official reports accurately assess the possibility of foul play, a number of alternative theories have been proposed. On 19 August 1998, the Archbishop Desmond Tutu, chairman of South Africa's Truth and Reconciliation Commission (TRC), stated that recently uncovered letters had implicated the British MI5, the American CIA, and then South African intelligence services in the crash. One TRC letter said that a bomb in the airplane's wheel bay was set to detonate when the wheels came down for a landing. Tutu said that they were unable to investigate the truth of the letters or the allegations that South Africa or Western intelligence agencies played a role in the crash. The British Foreign Office suggested that they may have been created as Soviet misinformation or disinformation. On 29 July 2005, the Norwegian Major General, Bjørn Egge, gave an interview to the newspaper Aftenposten on the events surrounding Hammarskjöld's death. According to General Egge, who had been the first UN officer to see the body, Hammarskjöld had a hole in his forehead, and this hole was subsequently airbrushed from photos taken of the body. It appeared to Egge that Hammarskjöld had been thrown from the plane, and grass and leaves in his hands might indicate that he survived the crash – and that he had tried to scramble away from the wreckage. Egge does not claim directly that the wound was a gunshot wound. In an interview on 24 March 2007, on the Norwegian TV channel NRK, an anonymous retired mercenary claimed that he had shared a room with an unnamed South African mercenary who claimed to have shot Hammarskjöld. The alleged killer was claimed to have died in the late 1990s. Given the evidence for bullet wounds on some of the crash victims and that the plane underwent an explosive fire, a possible scenario is that it was shot down either from the ground or by the Fouga reported in the area at the time.

In his speech to the 64th session of the United Nations General Assembly on 23 September 2009, Colonel Gaddafi called upon the Libyan president of UNGA, Ali Treki, to institute a UN investigation into the assassinations of Congolese prime minister, Patrice Lumumba, who was overthrown in 1960 and murdered the following year, and of UN Secretary-General Dag Hammarskjöld in 1961.