A former chairman of the Joint Chiefs of Staff, secretary of state and national security adviser, Mr. Powell died of complications of Covid-19, his family said.

Colin L. Powell, who in four decades of public life served as the nation’s top soldier, diplomat and national security adviser, and whose speech at the United Nations in 2003 helped pave the way for the United States to go to war in Iraq, died on Monday. He was 84.

The cause was complications of Covid-19, his family said in a statement, adding that he had been vaccinated and was being treated at Walter Reed National Military Medical Center, in Bethesda, Md., when he died there.

A spokeswoman said his immune system had been compromised by multiple myeloma, for which he had been undergoing treatment. He had been due to receive a booster shot for his vaccine last week, she said, but had to postpone it when he fell ill. He had also been treated for early stages of Parkinson’s disease, she said.

Mr. Powell was a pathbreaker, serving as the country’s first Black national security adviser, chairman of the Joint Chiefs of Staff and secretary of state. Beginning with his 35 years in the Army, Mr. Powell was emblematic of the ability of minorities to use the military as a ladder of opportunity.

His was a classic American success story. Born in Harlem of Jamaican parents, he grew up in the South Bronx and graduated from City College of New York, joining the Army through the R.O.T.C. Starting as a young second lieutenant commissioned in the dawn of a newly desegregated Army, Mr. Powell served two decorated combat tours in Vietnam. He was later national security adviser to President Ronald Reagan at the end of the Cold War, helping to negotiate arms treaties and an era of cooperation with the Soviet president, Mikhail S. Gorbachev.

As chairman of the Joint Chiefs, Mr. Powell was the architect of the invasion of Panama in 1989 and of the Persian Gulf war in 1991, which ousted Saddam Hussein from Kuwait but left him in power in Iraq. Along with Dick Cheney, the defense secretary at the time, Mr. Powell reshaped the American Cold War military that had stood ready at the Iron Curtain for half a century. In doing so he stamped the Powell Doctrine on military operations: Identify clear political objectives, gain public support and use decisive and overwhelming force to defeat enemy forces.

When briefing reporters at the Pentagon at the beginning of the gulf war, Mr. Powell summed up the military’s approach: “Our strategy in going after this army is very simple,” he said. “First, we’re going to cut it off, and then we’re going to kill it.”

It was a concept that seemed less well-suited to the messy conflicts in the Balkans that came later in the 1990s and in combating terrorism in a world transformed after the attacks on Sept. 11, 2001.

By the time he retired from the military in 1993, Mr. Powell was the most popular public figure in America, owing to his straightforwardness, his leadership qualities and his ability to speak in blunt tones that Americans appreciated.

In an interview with The New York Times in 2007, he analyzed himself in the third person: “Powell is a problem-solver. He was taught as a soldier to solve problems. So he has views, but he’s not an ideologue. He has passion, but he’s not a fanatic. He’s first and foremost a problem-solver.”

Once retired, Mr. Powell, a lifelong independent while in uniform, was courted as a presidential contender by both Republicans and Democrats, becoming America’s most political general since Dwight D. Eisenhower. He wrote a best-selling memoir, “My American Journey,” and flirted with a run for the presidency before deciding in 1995 that campaigning for office wasn’t for him.

He returned to public service in 2001 as secretary of state to President George W. Bush, whose father Mr. Powell had served as chairman of the Joint Chiefs a decade earlier.

But in the Bush administration Mr. Powell was the odd man out, fighting internally with Vice President Dick Cheney and Defense Secretary Donald H. Rumsfeld for the ear of President Bush and for foreign policy dominance.

He left at the end of Mr. Bush’s first term under the cloud of the ever-worsening war in Iraq begun after Sept. 11 and growing questions about whether he could, and should, have done more to object to it. Those questions swirled in part around his U.N. speech, which was based on false intelligence, and which became the source of lifelong regret.

He kept a low profile for the next few years, but with just over two weeks left in the 2008 presidential campaign, Mr. Powell, by now a declared Republican, gave a forceful endorsement to Senator Barack Obama, a Democrat, calling him a “transformational figure.” Mr. Powell’s backing was criticized by conservative Republicans. But it eased the doubts among some independents, moderates and even some moderates in his own party, and largely neutralized concerns about Mr. Obama’s lack of experience to be commander in chief.

When it came time to elect Mr. Obama’s successor, Mr. Powell continued his support of Democrats, saying he would vote for Hillary Clinton over Donald J. Trump. Before the election, he expressed disgust for Mr. Trump in a batch of leaked emails that a Powell spokesman confirmed as authentic.

“Trump is a national disgrace and an international pariah,” Mr. Powell wrote in one email. Mr. Trump’s attacks on whether Mr. Obama had been born in the United States also troubled him, the emails made clear. “Yup, the whole birther movement was racist,” he said.

In the next election, he backed Joseph R. Biden Jr., delivering a message of support for him at the 2020 Democratic National Convention.

Son of Immigrants
Colin Luther Powell was born on April 5, 1937, and reared in the ethnically mixed Hunts Point section of the South Bronx. His parents, Luther Powell, a shipping-room foreman in Manhattan’s garment district, and Maud Ariel McKoy, a seamstress, were immigrants from Jamaica.

The young Mr. Powell graduated from Morris High School in the Bronx. By his own account, he was a mediocre student, carrying a C average at the City College of New York as a geology major.

An early turning point came when he enrolled in the college’s Reserve Officer Training Corps program, drawn by the camaraderie it fostered, the discipline it imposed and its well-defined goals. Cadet Powell joined the Pershing Rifles, a drill team started by Gen. John J. Pershing, a top American commander in World War I. Even after becoming a general, Mr. Powell kept on his desk a pen set he had won for a drill-team competition decades earlier.

During a summer R.O.T.C. training tour in Fayetteville, N.C., in 1957, he got a pronounced taste of racism when he was forced to use segregated washrooms at gas stations in the South on the drive home to New York. After graduating from City College in June 1958, he was commissioned as a second lieutenant in the Army, the start of a 35-year military career.

He again experienced the still-segregated South during basic training at Fort Benning, Ga., before shipping out to Europe to become a platoon leader in West Germany in the Cold War.

Mr. Powell arrived in Saigon on Christmas Day 1962 for a one-year tour as an adviser to a 400-man South Vietnamese army battalion in the jungle. He completed the tour “a true believer” in the American effort, he later said, though the first inklings of skepticism toward the war were showing through.

While in the service, Mr. Powell met Alma Vivian Johnson on a blind date, and they married in August 1962. In addition to his wife, he is survived by two daughters, Linda Powell and Anne Powell Lyons; a son, Michael, who served as chairman of the Federal Communications Commission; and four grandchildren. He lived in McLean, Va.

(Mrs. Powell also tested positive for the coronavirus last week and was released from Walter Reed Military Medical Center after treatment, said Peggy Cifrino, a spokeswoman for the family. Mrs. Powell received two doses of the Moderna vaccine and will get her booster when it is approved, Ms. Cifrino said in an email.)

He arrived for his second tour in Vietnam in July 1968, serving as executive officer of an infantry battalion, then a division operations officer. Four months into his tour he had a brush with death when his helicopter crashed. He dragged his commander, Maj. Gen. Charles M. Gettys, out of the wreck, suffering a broken ankle.

Mr. Powell rose quickly through the ranks — including gaining a battalion command in Korea in 1973 and a brigade command in the elite 101st Airborne Division in 1976. He was tapped as a “water walker” by his peers, a term military men reserve for the most talented officers.

In 1979, Mr. Powell, then 42, was promoted to one-star general, becoming the youngest general officer in the Army at the time. After serving as Defense Secretary Casper Weinberger’s senior military assistant, Mr. Powell, in the spring of 1986, went off to command V Corps, skipping division command altogether in leading 75,000 soldiers in West Germany in the waning years of the Cold War. Just five months later, President Reagan summoned him back to Washington to be national security adviser, a post in which he played a pivotal role in helping to usher in a new era of cooperation with Mr. Gorbachev.

Mr. Powell left the White House in 1989 to return to lead the Army’s Forces Command; the promotion made him only the fourth Black four-star general in Army history. He saw himself not only as a model for Black soldiers but also as a challenge to white bigotry.

Mr. Powell had met Mr. Cheney when Mr. Cheney was a top House Republican leader and Mr. Powell was national security adviser. In his autobiography, he called Mr. Cheney “incisive, smart, no small talk, never showing any more surface than necessary” — a description that would come back to haunt Mr. Powell more than a decade later.

In October 1989, Mr. Powell succeeded Adm. William J. Crowe as chairman of the Joint Chiefs of Staff, leapfrogging over 14 more senior four-star officers. He was the first chairman to fully exercise power under the recently approved Goldwater-Nichols legislation, which made the chairman the “principal military adviser to the president and secretary of defense.”

Along with Mr. Cheney, General Powell presided over an active-duty military that had been cut in size by a quarter since its Cold War peak.

The Powell Doctrine
The promise of peace after the fall of the Iron Curtain was stopped short when Iraqi troops invaded Kuwait in August 1990. Mr. Powell urged caution and advocated imposing sanctions on Saddam Hussein’s regime rather than using military might. After President George H.W. Bush ordered the attack to drive Iraqi forces from Kuwait, Mr. Powell oversaw the military’s buildup of more than 500,000 troops in the Saudi desert.

The Powell doctrine was born out of the American military’s longstanding frustrations with the Vietnam War, in which the United States gradually escalated the use of force and declared periodic pauses in its bombing campaign. If American force is to be used, proponents of the doctrine said, it should be overpowering and decisive.

The purest examples of the Powell doctrine were the 1991 war with Iraq and the 1989 invasion of Panama, when the United States military stormed the country in a several-day blitz and captured its leader, Manuel Noriega.

Mr. Powell’s relationship with Mr. Cheney was professional but distant. “He and I had never, in nearly four years, spent a single purely social hour together,” Mr. Powell wrote. (Years later, that prickly relationship resurfaced when Mr. Powell and Mr. Cheney clashed in the White House of President George W. Bush. After Mr. Cheney, in a 2011 memoir, wrote that Mr. Powell had felt more comfortable expressing his views about Iraq to the public than to President Bush, Mr. Powell criticized him for taking “cheap shots.”)

Under the newly elected President Bill Clinton, Mr. Powell and other members of the Joint Chiefs confronted him over his promise to allow gay men and lesbians to serve openly in the armed forces, a tense debate that led to the “don’t ask, don’t tell” compromise policy that lasted until 2011, when they were indeed allowed to serve openly.

Mr. Powell resisted using force against Bosnian Serbs in 1993. And he supported leaving a weakened Saddam Hussein in place as a bulwark against Iran and Syria. If Mr. Hussein was overthrown, Mr. Powell worried, someone worse would take his place.

In his final days as chairman in 1993, 18 Americans troops were killed pursuing a warlord in Mogadishu, Somalia, in an incident that became know as “Blackhawk Down” after two transport helicopters were shot down. A Senate Armed Services Committee inquiry blamed Mr. Powell and Les Aspin, the defense secretary at the time, for leaving Army Rangers without sufficient protection.

After retiring in October 1993, Mr. Powell reportedly received a $6 million advance to write his memoirs. “My American Journey,” written with Joseph E. Persico, was released in September 1995 and was an immediate best seller.

As he flirted for a run for the presidency, huge crowds turned out for his book signings in 1995.

Mr. Powell agonized over the decision. He drafted two speeches in October 1995, one announcing he would run for the White House, with plans to deliver it at City College, and the other saying he would not. Finally, on Nov. 8, 1995, resisting an enormous popular drumbeat for his candidacy, he said he did not have the drive and desire necessary to run for the nation’s highest office.

But he announced he would register as a Republican (after previously registering as an independent), and he spoke at the 1996 and 2000 Republican National Conventions. Instead of focusing on national security or military affairs, however, he spoke on broader domestic policy issues, including the rights of minorities and children’s welfare.

Mr. Powell had learned at the feet of political masters — Cap Weinberger and Frank Carlucci. Mr. Powell was military assistant to Mr. Carlucci when Mr. Carlucci was the Pentagon’s second-ranking official; he served in the same position when Mr. Weinberger was defense secretary. (Mr. Carlucci succeeded Mr. Weinberger as defense secretary under Reagan.)

“Anybody who becomes a senior officer had better have some political instincts or you’re going to get ground up,” Mr. Powell said in the 2007 Times interview. “We are a political nation. It is not a dirty word.”

Secretary of State
He returned to government in December 2000, when he was the first person appointed to the cabinet of President-elect George W. Bush.

His assignment, as secretary of state, started with high hopes and soaring rhetoric from his new boss. “General Powell is an American hero, an American example and a great American story,” Mr. Bush said in announcing his choice on Dec. 16, 2000. “It’s a great day when a son of the South Bronx succeeds to the office first held by Thomas Jefferson.”

Mr. Powell reinvigorated a demoralized department. A computer buff, he brought 21st-century technology to the department and its far-flung embassies. He ordered high-speed internet into overseas posts and expanded training for senior Foreign Service officers and political appointees assigned abroad.

Mr. Powell already had a taste of the diplomatic world when he was national security adviser and chairman of the Joint Chiefs. And after he retired from the military, President Clinton dispatched him, Senator Sam Nunn and former President Jimmy Carter in September 1994 to stave off a potentially bloody invasion of Haiti. The three reached a last-minute agreement with the ruling military junta, allowing a peaceful landing of American forces to help stabilize the country amid unrest.

Mr. Clinton had sounded out Mr. Powell for the secretary of state job in his administration, but Mr. Powell declined. But when George W. Bush asked him to take the post nearly a decade later, Mr. Powell jumped at the opportunity. He had known Mr. Bush, whom he had called “Sonny,” when his father was president. But he had only passing acquaintance with him before the 2000 campaign. Indeed, many political advisers in the Bush campaign were initially wary of the appointment because Mr. Powell had indicated that he might support the Republican candidacy of his old friend, Senator John McCain, in the 2000 primaries.

Mr. Powell took the job at Foggy Bottom without having extensive conversations with Mr. Bush about the president’s views and what Mr. Bush expected. In the first few months of the new administration, he was forced to reverse his publicly stated goal of engaging with North Korea’s hermetic regime and to adopt the White House’s more confrontational approach to preventing the North from developing nuclear weapons.

He clashed with Mr. Cheney and some of the more conservative members of Mr. Bush’s foreign policy team, and was slow to discern his isolation from the rest of the national security team. “The years at State were quite difficult,” said William H. Taft IV, a longtime friend who served as the State Department’s legal adviser during Mr. Bush’s first term. “They were with people who had different world views.”

The terrorist attacks on Sept. 11, 2001, put the country on a war footing and galvanized Mr. Bush’s war council. But eight months after the Taliban regime in Afghanistan had fallen, the United States was secretly planning another war: to oust Mr. Hussein from Iraq once and for all.

Mr. Powell was a reluctant warrior in this impending fight, warning Mr. Bush that invading Iraq could destabilize the Middle East, upset oil markets and divert political will and resources from the unfinished fight against Al Qaeda. In a two-hour meeting with Mr. Bush on Aug. 5, 2002, Mr. Powell laid down what became known as the Pottery Barn rules: “You break it, you’re going to own it.”

As he did in the Persian Gulf war, Mr. Powell did not recommend whether the country should go to war or not — that, he believed, was the president’s prerogative alone — but he outlined options. After a failed diplomatic effort to avert a conflict, Mr. Bush turned to Mr. Powell to bolster the administration’s case for use of force if Mr. Hussein did not comply with international demands.

A ‘Painful’ Speech at the U.N.

In a 76-minute speech at the United Nations on Feb. 5, 2003, Mr. Powell pressed the American case for a possible war to disarm Iraq, presenting photographs, electronic intercepts of conversations between Iraqi military officers and information from defectors aimed at proving that Mr. Hussein posed an imminent danger to the world.

In the Bush administration’s most explicit effort to connect the activities between Iraq and Al Qaeda, Mr. Powell suggested that Iraq’s lethal weapons could be given at any time to terrorists who could use them against the United States or Europe.

He provided new details about what he said were Iraq’s effort to develop mobile laboratories to make germ weapons. He asserted that Iraq had sought to hide missiles in its western desert. Significantly, he cited intelligence reports that Mr. Hussein had authorized his military to use poison gas if the United States invaded.

Before the speech, Mr. Powell had spent several days at the C.I.A. grilling analysts on the intelligence, paring back many of the claims in an early White House draft of the speech that he felt were unsupported. Now he felt confident, he told aides before the address in New York.

“Leaving Saddam Hussein in possession of weapons of mass destruction for a few more months or years is not an option, not in a post-September 11th world,’’ Mr. Powell declared.

The speech failed to persuade many skeptics in the international community, but Mr. Powell’s personal appeal swung many Americans to support the war, however reluctantly. After American troops invaded in March 2003, however, it became clear that there were no weapons of mass destruction. The intelligence had been wrong.

Two years later, Mr. Powell told Barbara Walters of ABC News that his speech to the United Nations had been “painful” for him personally and would forever be a “blot” on his record.

“I’m the one who presented it on behalf of the United States to the world,” Mr. Powell said, acknowledging that his presentation “will always be a part of my record.”

He left office in January 2004, returning to life as a private citizen. In 1997, he had founded America’s Promise, a nonprofit organization dedicated to helping at-risk children. He later served as the chairman of the board of visitors of the School for Civic and Global Leadership, named for him at the City University of New York.

Years later, the sting of the United Nations speech still pained him. Yet he sought to move on. “Let others judge me,” Mr. Powell said in the 2007 interview. “All I want to do is judge myself as a successful soldier who served his best.”