Kofi Annan, the soft-spoken and patrician diplomat from Ghana who became the seventh secretary general of the United Nations, projecting himself and his organization as the world’s conscience and moral arbiter despite bloody debacles that stained his record as a peacekeeper, died on Saturday in Bern, Switzerland. He was 80.

His death, at a hospital there, was confirmed by his family in a statement released by the Kofi Annan Foundation, which is based in Switzerland. It said he died after a short illness but did not specify the cause.

Awarded the Nobel Peace Prize in 2001, Mr. Annan was the first black African to head the United Nations, doing so for two successive five-year terms beginning in 1997 — a decade of turmoil that challenged that sprawling body and redefined its place in a changing world.

On his watch as what the Nobel committee called Africa’s foremost diplomat, Al Qaeda struck New York and Washington, the United States invaded Iraq, and Western policymakers turned their sights from the Cold War to globalization and the struggle with Islamic militancy.

An emblem as much of the United Nations’ most ingrained flaws as of its grandest aspirations, Mr. Annan was the first secretary general to be chosen from among the international civil servants who make up the organization’s bureaucracy.

He came to be likened in stature to Dag Hammarskjold, the second secretary general, who died in a mysterious plane crash in Africa in 1961. Mr. Annan was credited with revitalizing the United Nations’ institutions, shaping what he called a new “norm of humanitarian intervention,” particularly in places where there was no peace for traditional peacekeepers to keep.

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And, not least, he was lauded for persuading Washington to unblock arrears that had been withheld because of the profound misgivings about the United Nations voiced by American conservatives.

His tenure was rarely free of debate, however. In 1998, Mr. Annan traveled to Baghdad to negotiate directly with Saddam Hussein over the status of United Nations weapons inspections, winning a temporary respite in the long battle of wills with the West but raising questions about his decision to shake hands — and even smoke cigars — with that dictator.

In fact, Mr. Annan called the 2003 invasion of Iraq illegal and suffered an acute personal loss when a trusted and close associate, the Brazilian official Sérgio Vieira de Mello, his representative in Baghdad, died in a suicide truck bombing in August 2003 that struck the United Nations office there, killing many civilians.

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The attack prompted complaints that Mr. Annan had not grasped the perils facing his subordinates after the ouster of Mr. Hussein.

While his admirers praised his courtly, charismatic and measured approach, Mr. Annan was hamstrung by the inherent flaw of his position as what many people called a “secular pope” — a figure of moral authority bereft of the means other than persuasion to enforce the high standards he articulated.

As secretary general, Mr. Annan, like all his predecessors and successors, commanded no divisions of troops or independent sources of income. Ultimately, his writ extended only as far as the usually squabbling powers making up the Security Council — the highest U.N. executive body — allowed it to run.

In his time, those divisions deepened, reaching a nadir in the invasion of Iraq. Over his objections, the campaign went ahead on the American and British premise that it was meant to disarm the Iraqi regime of chemical weapons, which it did not have or, at least, were never found.

Iraq also brought embarrassment closer to home when reports began surfacing in 2004 that Mr. Annan’s son, Kojo Annan, worked for Cotecna Inspection Services, a Geneva-based company that had won a lucrative contract in a vast humanitarian program, known as oil for food, that was supervised by the United Nations in Iraq.

A commission led by Paul A. Volcker, the former chairman of the Federal Reserve, concluded that the secretary general had not influenced the awarding of the contract but also that he had not investigated it aggressively once questions were raised.

Mr. Annan said he took the commission’s findings as exoneration, but his reputation suffered, particularly in the eyes of adversaries in Washington.

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Times of Bloodshed
In assessing his broader record, moreover, many critics singled out Mr. Annan’s personal role as head of the United Nations peacekeeping operations from 1993 to 1997 — a period that saw the killing of 18 American service personnel in Somalia in October 1993, the deaths of more than 800,000 Rwandans in the genocide of 1994, and the massacre of 8,000 Bosnian Muslims by Bosnian Serb forces at Srebrenica in 1995.

In Rwanda and Bosnia, United Nations forces drawn from across the organization’s member states were outgunned and showed little resolve. In both cases, troops from Europe were quick to abandon their missions. And in both cases, Mr. Annan was accused of failing to safeguard those who had looked to United Nations soldiers for protection.

“Annan felt that the very countries that had turned their backs on the Rwandans and Bosnians were the ones making him their scapegoat,” Samantha Power, an author who became the United States ambassador to the United Nations during the Obama administration, wrote in 2008. “But he knew that his name would appear in the history books beside the two defining genocidal crimes of the second half of the 20th century.”

Despite the serial setbacks, Mr. Annan commanded the world stage with ease in his impeccably tailored suits, goatee and slight, graceful physique — attributes that made him and his second wife, Nane Lagergren, a global power couple.

He seemed to radiate an aura of probity and authority. “How do we explain Kofi Annan’s enduring moral prestige,” the Canadian author, politician and academic Michael Ignatieff wrote in a review of Mr. Annan’s 2012 memoir, “Interventions: A Life in War and Peace” (with Nader Mousavizadeh), in The New York Review of Books.

“Personal charisma is only part of the story,” Mr. Ignatieff wrote. “In addition to his charm, of which there is plenty, there is the authority that comes from experience. Few people have spent so much time around negotiating tables with thugs, warlords and dictators. He has made himself the world’s emissary to the dark side.”

The desire to burnish his legacy seemed to motivate Mr. Annan long after Ban Ki-moon replaced him as secretary general, and he set up a nonprofit foundation to promote higher standards of global governance. In 2008, he headed a commission of eminent Africans that persuaded rival factions in Kenya to reconcile a year after more than 1,000 people were killed during and after disputed elections.

In February 2012, Mr. Annan was appointed as the joint envoy of the Arab League and the United Nations to seek a settlement in Syria as civil war there tightened its grip. But he resigned in frustration that August, citing the intransigence of both sides in a conflict that had convulsed and reshaped the region and claimed hundreds of thousands of lives.

After Mr. Annan’s death, Bijan Farnoudi, a spokesman for the Kofi Annan Foundation, said in an email that Mr. Annan had lived in Geneva for the past decade, running the nonprofit. He had recently returned from a working trip to Zimbabwe “a little weakened,” Mr. Farnoudi said, “but all those working closely with him day in and day out did not see this coming.”

“He worked until the very end, without giving himself a break,” he said. “And he looked strong and fit doing it.”

Kofi Atta Annan was born on April 8, 1938, in the city of Kumasi in what was then Gold Coast and which, in 1957, became Ghana, the first African state to achieve independence from British colonialism. Born into an aristocratic family, he had three sisters, two of them older. The third, Efua, was a twin who died in the 1990s.

After a spell at the elite Mfantsipim boarding school founded by Methodists, he went on to higher education as an economist in Ghana, at Macalester College in St. Paul, in Geneva, and at the Sloan School of Management at the Massachusetts Institute of Technology.

In 1965, he married Titi Alakija, a woman from a prosperous Nigerian family. The couple had two children, a daughter, Ama, and a son, Kojo. The marriage foundered in the late 1970s.

Mr. Annan married Ms. Lagergren, a divorced lawyer working at the United Nations, in 1984. She, too, was a scion of a prominent family, a niece of Raoul Wallenberg, the Swedish diplomat who protected thousands of Hungarian Jews during World War II but disappeared after being captured by Soviet forces. Ms. Lagergren had a daughter, Nina, from her first marriage. He is survived by Ms. Lagergren, his children and a brother, Kobina, a former Ghanaian ambassador to Morocco.

Most of Mr. Annan’s working life was spent in the corridors and conference rooms of the United Nations, but, he told the author Philip Gourevitch in 2003, “I feel profoundly African, my roots are deeply African, and the things I was taught as a child are very important to me.”

His first appointment with a United Nations agency was in 1962, at the World Health Organization in Geneva. Mr. Annan returned briefly to Ghana to promote tourism and worked in Ethiopia with the U.N. Economic Commission for Africa before returning to the health organization’s European headquarters.

Peacekeeper in Chief
Later, in New York, he worked in senior human resources and budgetary positions until, in the early 1990s, the secretary general at the time, Boutros Boutros Ghali of Egypt, appointed him first as deputy and then as head of peacekeeping operations.

The appointment plunged Mr. Annan into a maelstrom of conflicts in which United Nations forces were deployed. As genocide approached Rwanda in 1994 — months after the downing of a Black Hawk helicopter in Mogadishu, Somalia, and the killing of American service personnel — the Clinton administration in Washington had little appetite for intervention.

But on the ground, the Canadian commander, Lt. Gen. Roméo Dallaire, heading a modest force of 2,500 United Nations troops, sought permission from Mr. Annan’s office to raid an arms cache that he believed would be used in massacres. Permission was refused. Only years later, after the release of a critical report in 1999, did Mr. Annan declare that “all of us must bitterly regret that we did not do more to prevent it.”

“On behalf of the United Nations, I acknowledge this failure and express my deep remorse,” he said.

In Bosnia, too, the United Nations was accused of being overcautious. Critics said it had been restricted by a mandate, approved by the Security Council, for the establishment of so-called safe havens under United Nations protection that proved, in Srebrenica, to be illusory. European powers opposed airstrikes to halt the advancing Bosnian Serbs, who overran Srebrenica despite the presence of peacekeeping troops from the Netherlands.

Later that year, Mr. Annan seemed to adopt a tougher line, approving the NATO bombing campaign that forced Serbia to the negotiating table to sign the 1995 Dayton peace accords, so named because the negotiations had taken place at the Wright-Patterson Air Force Base in Ohio. At that time, airstrikes required a so-called dual key approval of the NATO command and the United Nations.

“When Kofi turned it,” Richard C. Holbrooke, the former American envoy, told Mr. Gourevitch, “he became secretary general in waiting.”

With Washington pressing for the ouster of Mr. Boutros Ghali, Mr. Annan took office as secretary general with American approval on Jan. 1, 1997.

He was, Ms. Power wrote, “the primary guardian of the U.N. rule book,” which insisted on the primacy of the Security Council as “the sole source of legitimacy,” in Mr. Annan’s words, in approving overseas interventions. Those rules were flouted by NATO in March 1999, with its bombing of the former Yugoslavia, forcing Mr. Annan to seek a middle ground.

“It is, indeed, tragic that diplomacy has failed,” he said on the first day of NATO bombing, choosing words that largely defined the dilemmas confronting policymakers throughout and beyond his tenure, “but there are times when the use of force may be legitimate in the pursuit of peace.”

“We will not, and we cannot accept a situation where people are brutalized behind national boundaries,” he continued later as the 78-day aerial campaign ended its second week of efforts to halt a crackdown on ethnic Albanians in Kosovo.

“For at the end of the 20th century, one thing is clear: A United Nations that will not stand up for human rights is a United Nations that cannot stand up for itself.”

Correction: Aug. 23, 2018
An earlier version of this obituary omitted the name of a survivor. In addition to his wife and children, Mr. Annan is survived by a brother, Kobina, a former Ghanaian ambassador to Morocco.