U.N. Secretary-General Dag Hammarskjöld dies in an air crash while attempting to negotiate peace in the Katanga region of the Democratic Republic of the Congo.

Dag Hjalmar Agne Carl Hammarskjöld was a Swedish economist and diplomat who served as the second secretary-general of the United Nations from April 1953 until his death in a plane crash in September 1961. As of 2025, he remains the youngest person to have held the post, having been only 47 years old when he was elected. He was a son of Hjalmar Hammarskjöld, who served as Prime Minister of Sweden from 1914 to 1917.

On 18 September 1961, a DC-6 passenger aircraft of Transair Sweden operating for the United Nations (UN) crashed near Ndola, Northern Rhodesia. The crash resulted in the deaths of all people on board, including Dag Hammarskjöld, the second secretary-general of the United Nations, and 15 others. Hammarskjöld had been en route to ceasefire negotiations with Moïse Tshombe during the Congo Crisis. Three official inquiries failed to conclusively determine the cause. Some historians and military experts like Susan Williams have criticized the official inquiries, pointing to evidence of foul play that had been omitted from the inquiries.