Dag Hammarskjold in 1955
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September 25, 2015 11:10 AM EDT

In his speech at the United Nations Friday morning, Pope Francis turned again and again to the international organization’s hitting its 70th anniversary and its long history of attempting to work toward peace. And, though that history stretches over many decades, there was one former U.N. Secretary-General who the Pope called out by name: Dag Hammarskjöld, who held the position in the 1950s. As the Pope explained, he gave his life “for peace and reconciliation among peoples.”

So what happened to Hammarskjöld?

The Swedish-born diplomat, who became Secretary-General in 1953, was killed in a plane accident in Africa, while visiting the Congo for a peace mission. The occasion for the visit was the political crisis in the Congo that had begun the year before. After the African nation gained its independence from Belgium, war erupted over who would lead the nation, as Cold War-era proxy fighting drew the world’s eyes to its future. U.N. peacekeepers were involved to an extent that the organization had never before seen, as they tried to prevent full scale civil war. In the relatively early days of the operation, in September of 1961, Hammarskjöld flew to Léopoldville to personally check out what was going on.

Though war seemed imminent, the accomplished diplomat managed to convince the leaders on the two sides to discuss a ceasefire. He would personally lead the meeting, even though its location, in the city of Ndola, meant that his plane would have to cross dangerous territory. Though precautions were taken, Hammarskjöld never arrived at his destination. In the confusion that followed, it took nine hours for a search party to go out. Late in the afternoon after he was scheduled to get to Ndola, his plane was found, crashed, in a forest mere miles from the airport. There was only one survivor, a security guard who died a few days later. Though it was suspected that the crash was the work of a well-known partisan, authorities soon concluded that Hammarskjöld’s plane had probably malfunctioned, though it would remain impossible to say for sure.

As TIME explained the week after his death, Hammarskjöld had an outsized influence in revitalizing the United Nations:

His death threatened to derail the United Nations, suddenly left without a leader at a time when Cold War tensions characterized international relations between the major powers. The U.N.’s founders had, oddly enough, not left written instructions for how to proceed should a Secretary-General end his term due to death. It seemed to many observers that the loss of Hammarskjöld would mean the loss of that hope he had fostered, the hope for a world where small steps toward peace seemed possible. “As though I had lost a dear and personal friend, I wept bitter tears of sorrow and regret at the news of the death of a great and good man, a true and courageous gentleman, Dag Hammarskjöld,” one TIME reader wrote to the magazine in the days that followed. “He belonged not to his native land alone but to all the free world.”

And yet, more than a half-century later, the United Nations continues to trudge ahead. For a speech in which the Pope also mentioned that “the experience of the past 70 years has made it clear that reform and adaptation to the times is always necessary,” few names would be more relevant to invoke than Hammarskjöld’s.

Read more about the aftermath of Hammarskjold’s death, here in the TIME Vault: The U.N. in Crisis

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Write to Lily Rothman at lily.rothman@time.com.

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