Robert S. McNamara, the forceful and cerebral defense secretary who helped lead the nation into the maelstrom of Vietnam and spent the rest of his life wrestling with the war’s moral consequences, died Monday at his home in Washington. He was 93.

His wife, Diana, said Mr. McNamara died in his sleep at 5:30 a.m., adding that he had been in failing health for some time.

Mr. McNamara was the most influential defense secretary of the 20th century. Serving Presidents John F. Kennedy and Lyndon B. Johnson from 1961 to 1968, he oversaw hundreds of military missions, thousands of nuclear weapons and billions of dollars in military spending and foreign arms sales. He also enlarged the defense secretary’s role, handling foreign diplomacy and the dispatch of troops to enforce civil rights in the South.

“He’s like a jackhammer,” Johnson said. “No human being can take what he takes. He drives too hard. He is too perfect.”

As early as April 1964, Senator Wayne Morse, Democrat of Oregon, called Vietnam “McNamara’s War.” Mr. McNamara did not object. “I am pleased to be identified with it,” he said, “and do whatever I can to win it.”

Half a million American soldiers went to war on his watch. More than 16,000 died; 42,000 more would fall in the seven years to come.

The war became his personal nightmare. Nothing he did, none of the tools at his command — the power of American weapons, the forces of technology and logic, or the strength of American soldiers — could stop the armies of North Vietnam and their South Vietnamese allies, the Vietcong. He concluded well before leaving the Pentagon that the war was futile, but he did not share that insight with the public until late in life.

In 1995, he took a stand against his own conduct of the war, confessing in a memoir that it was “wrong, terribly wrong.” In return, he faced a firestorm of scorn.

“Mr. McNamara must not escape the lasting moral condemnation of his countrymen,” The New York Times said in a widely discussed editorial, written by the page’s editor at the time, Howell Raines. “Surely he must in every quiet and prosperous moment hear the ceaseless whispers of those poor boys in the infantry, dying in the tall grass, platoon by platoon, for no purpose. What he took from them cannot be repaid by prime-time apology and stale tears, three decades late.”

By then he wore the expression of a haunted man. He could be seen in the streets of Washington — stooped, his shirttail flapping in the wind — walking to and from his office a few blocks from the White House, wearing frayed running shoes and a thousand-yard stare.

He had spent decades thinking through the lessons of the war. The greatest of these was to know one’s enemy — and to “empathize with him,” as Mr. McNamara explained inErrol Morris’s 2003 documentary, “The Fog of War: Eleven Lessons from the Life of Robert S. McNamara.”

“We must try to put ourselves inside their skin and look at us through their eyes,” he said. The American failure in Vietnam, he said, was seeing the enemy through the prism of the cold war, as a domino that would topple the nations of Asia if it fell.

In the film, Mr. McNamara described the American firebombing of Japan’s cities in World War II. He had played a supporting role in those attacks, running statistical analysis for Gen. Curtis E. LeMay of the Army’s Air Forces.

“We burned to death 100,000 Japanese civilians in Tokyo — men, women and children,” Mr. McNamara recalled; some 900,000 Japanese civilians died in all. “LeMay said, ‘If we’d lost the war, we’d all have been prosecuted as war criminals.’ And I think he’s right. He — and I’d say I — were behaving as war criminals.”

“What makes it immoral if you lose and not immoral if you win?” he asked. He found the question impossible to answer.

From Detroit to Washington

The idea of the United States’ losing a war seemed impossible when Mr. McNamara came to the Pentagon in January 1961 as the nation’s eighth defense secretary. He was 44 and had been named president of the Ford Motor Company only 10 weeks before. He later said, half-seriously, that he could barely tell a nuclear warhead from a station wagon when he arrived in Washington.

“Mr. President, it’s absurd; I’m not qualified,” he remembered protesting when asked to serve. He said that Kennedy had replied, “Look, Bob, I don’t think there’s any school for presidents, either.”

Kennedy called him the smartest man he had ever met. Mr. McNamara looked steely-eyed and supremely rational behind his wire-rimmed glasses, his brown hair slicked back precisely and crisply parted on top. Mr. McNamara had risen by his mastery of systems analysis, the business of making sense of large organizations — taking on a big problem, studying every facet, finding simplicity in the complexity.

His first mission was to defuse the myth of the missile gap. Kennedy had argued in his 1960 presidential campaign that the strategic nuclear arsenal of the United States was less powerful than the Soviet Union’s, and that the gap was growing. His predecessor as president, Dwight D. Eisenhower, called the missile gap a fiction in his final State of the Union address, on Jan. 12, 1961.

Mr. McNamara took office nine days later. He recalled that “my first responsibility as secretary of defense was to determine the degree of the gap and initiate action to close it.”

“It took us about three weeks to determine, yes, there was a gap,” he told an oral historianat his alma mater, the University of California, Berkeley. “But the gap was in our favor. It was a totally erroneous charge that Eisenhower had allowed the Soviets to develop a superior missile force.”

The problem was a lack of accurate intelligence; the estimate of Soviet forces had been a product of politics and guesswork.

By year’s end, new American spy satellites had determined that the Soviets had as few as 10 launchers from which missiles could be fired at the United States, while the United States could strike with more than 3,200 nuclear weapons.

At the same time, Mr. McNamara was enmeshed in plans for the Bay of Pigs invasion, in which some 1,500 Cubans, trained and equipped by the Central Intelligence Agency, were badly defeated by Fidel Castro’s forces in a bloody battle in April 1961. Mr. McNamara doubted that the C.I.A.’s Cubans could overthrow Mr. Castro, who had taken power in 1959, but he asked few questions beforehand and gave his go-ahead to the plan, which had been conceived under the Eisenhower administration.

Kennedy’s first order to Mr. McNamara after the invasion of Cuba collapsed was to develop a proposal for overthrowing the Castro government with American military force. Ten days later, he submitted a plan of attack that included 60,000 American troops, excluding naval and air forces. The plan proved impossible to fulfill.

One lesson of the Bay of Pigs, Mr. McNamara told the Joint Chiefs of Staff, was that “the government should never start anything unless it could be finished, or the government was willing to face the consequences of failure,” according to the State Department’s official record of American foreign policy, “The Foreign Relations of the United States.”

At a White House meeting on Nov. 3, 1961, Kennedy authorized a program designed to undermine the Castro government, code-named Operation Mongoose. Attorney GeneralRobert F. Kennedy’s handwritten notes on the meeting say Mr. McNamara was assigned to survey the situation and help him devise ways “to stir things up on island with espionage, sabotage, general disorder.” This operation also failed.

By 1962, the White House and the Pentagon had devised a new strategy of counterinsurgency to combat what Mr. McNamara called the tactics of “terror, extortion and assassination” by communist guerrillas. The call led to the creation of American special forces like the Green Berets and secret paramilitary operations throughout Asia and Latin America.

“Counterinsurgency became an almost ridiculous battle cry,” said Robert Amory, who in 1962 stepped down after nine years as the C.I.A.’s deputy director of intelligence to become the White House budget officer for classified programs.

While the United States flailed at Cuba, the Soviet Union decided, in the words of its leader, Nikita S. Khrushchev, “to throw a hedgehog at Uncle Sam’s pants.” It began sending nuclear missiles to Cuba, establishing a direct threat that evened up the balance of power with the United States, which had placed its own missiles near the Soviet border in Turkey.

At the height of the missile crisis, on Oct. 27, 1962, the Joint Chiefs of Staff recommended that Cuba be invaded within 36 hours. As the secret White House taping system installed by Kennedy recorded his words, Mr. McNamara laid out the prospects for war.

“The military plan is basically invasion,” he said. “When we attack Cuba, we are going to have to attack with an all-out attack.”

He continued, “The Soviet Union may, and, I think, probably will, attack the Turkish missiles.” The United States would then have to attack Soviet ships or bases in the Black Sea, he said. The chances of an uncontrolled escalation were high.

“And I would say that it is damn dangerous,” he said. “Now, I’m not sure we can avoid anything like that if we attack Cuba. But I think we should make every effort to avoid it. And one way to avoid it is to defuse the Turkish missiles before we attack Cuba.”

That idea — a secret deal in which Kennedy offered to withdraw his missiles in Turkey if Khrushchev removed his warheads from Cuba — resolved the crisis. “In the end, we lucked out — it was luck that prevented nuclear war,” Mr. McNamara said in “The Fog of War,” 40 years after the fact.

Mr. McNamara spent countless hours as secretary of defense trying to fine-tune American plans for nuclear war, turning what had been a hair-trigger, all-or-nothing strategy into a series of more limited options. The underlying principle of nuclear deterrence became known as “mutual assured destruction” — meaning that Washington and Moscow each knew it could destroy the other even if the other struck first.

In retirement, Mr. McNamara argued that planning for nuclear war was futile. “Nuclear weapons serve no military purposes whatsoever,” he wrote. “They are totally useless — except only to deter one’s opponent from using them.”

He had come close to that conclusion after the Cuban missile crisis. “In wars prior to the advent of nuclear weapons, damage was reparable and victory attainable,” Mr. McNamara said on Dec. 14, 1962, in a speech to NATO foreign ministers in Paris. “But after a full nuclear exchange such as the Soviet bloc and the NATO alliance are now able to carry out, the fatalities might well exceed 150 million.”

“The devastation would be complete and victory a meaningless term,” he said.

Remaking the Pentagon

“This place is a jungle, a jungle,” Mr. McNamara said after a few weeks at his desk at the Pentagon. He sent teams of bright young civilians — the whiz kids, as they were known — out across the Pentagon to tame it.

They set out to make sense of a cacophony of war strategies, weapons systems and budgets among the Army, the Navy and the Air Force. The office of the secretary of defense had been established in 1947 for precisely that purpose, but the task had defeated everyone who held the job before Mr. McNamara. He applied the tools of systems analysis and succeeded in clearing some swaths through the jungle. But he alienated key members of Congress and military commanders in battles over choosing weapons and closing bases.

The Pentagon consumed nearly half the national budget when he took office. He had 3.5 million employees — including 2.5 million in uniform, a number that increased by a million during his tenure. He said his goal was “to bring efficiency to a $40 billion enterprise beset by jealousies and political pressures.”

Under Mr. McNamara, the Pentagon’s budget increased to $74.9 billion in fiscal 1968, from $48.4 billion in 1962. The 1968 figure is equal to $457 billion in today’s dollars.

That was largely the cost of the war that erupted in Southeast Asia.

“Every quantitative measurement we have shows we are winning this war,” Mr. McNamara said after returning from his first trip to South Vietnam in April 1962. His statistical analysis showed that the military mission could be wrapped up in three or four years.

After Kennedy was assassinated on Nov. 22, 1963, Mr. McNamara found that Johnson depended on him to win the war, which became a full-fledged conflict for the United States the following year. The new president thought so highly of Mr. McNamara that he asked him to be his running mate in 1964.

“I said no,” Mr. McNamara recounted in his Berkeley oral history. “You shouldn’t start your elective career running for the vice presidency.” (Johnson chose Senator Hubert H. Humphrey of Minnesota.)

Johnson relied on Mr. McNamara in other sensitive matters, including negotiations over weapons sales to Israel and the full racial integration of the armed services, the reserves and the National Guard after the passage of the Civil Rights Act of 1964. When Johnson, early in his presidency, announced he wanted to keep the federal budget below $100 billion, Mr. McNamara ordered weapons programs canceled and military bases closed in a matter of days.

But by the fall of 1964, Vietnam was the all-consuming obsession.

Congress authorized the war after Johnson contended that American warships had been attacked by North Vietnamese patrol boats in the Gulf of Tonkin on Aug. 4, 1964. The attack never happened, as a report declassified by the National Security Agency in 2005 made clear. The American ships had been firing at radar shadows on a dark night.

At the time, however, the agency’s experts in signals intelligence, or sigint, told Mr. McNamara that the evidence of an attack was iron-clad. “McNamara had taken over raw sigint and shown the president what they thought was evidence,” said Ray Cline, then the C.I.A.’s deputy director of intelligence. He added, “It was just what Johnson was looking for.”

Nor was this the only case of faulty intelligence underlying American military action under Mr. McNamara. In April 1965, Johnson ordered 24,000 American troops to the Dominican Republic after a revolt against the government; it was the first large-scale American landing in Latin America since 1928.

In public, Mr. McNamara said the deployment had showed the “readiness and capabilities of the U.S. defense establishment to support our foreign policy.” In private, he voiced dismay. The C.I.A. had told the White House and the Pentagon that the rebels were controlled by Cuban revolutionaries. But Mr. McNamara had deep doubts.

“You don’t think C.I.A. can document it?” Johnson asked him, according to tapes of White House telephone conversations recorded on April 30, 1965.

“I don’t think so, Mr. President,” McNamara replied. “I just don’t believe the story.”

Johnson nonetheless insisted in a speech to the American people that he would not allow “Communist conspirators” to establish “another Communist government in the Western Hemisphere.” This led some newspapers to assert that the president and the Pentagon had a “credibility gap.” The phrase stuck when applied to Vietnam.

Turning on Vietnam

In 1965, tens of thousands of American combat troops were arriving in Vietnam and American warplanes were pounding the enemy in a bombing campaign code-named Rolling Thunder, which sent 55,000 flights with 33,000 tons of bombs over North Vietnam; the next year, it was 148,000 flights with 128,000 tons. The number of aircraft lost went from 171 in 1965 to 318 the next year; the costs soared to $1.2 billion, from $460 million.

Rolling Thunder never stopped the flow of enemy arms and soldiers into South Vietnam.

When Mr. McNamara held a rare private briefing for reporters in Honolulu in February 1966, he no longer possessed the radiant confidence he had always displayed in public. Mr. McNamara said with conviction, “No amount of bombing can end the war.”

By 1966, Mr. McNamara was planning to build an electronic barrier across the demilitarized zone that separated North and South Vietnam. Soldiers called it the McNamara Line, after the Maginot Line, a futile French defense against Germany built before World War II. The barrier proved to be worthless.

On Aug. 26, 1966, Mr. McNamara read a book-length C.I.A. study called “The Vietnamese Communists’ Will to Persist,” which concluded that nothing the United States was doing could defeat the enemy. He called in a C.I.A. analyst, George Allen, who had spent 17 years working on the question of Vietnam.

“He wanted to know what I would do if I were sitting in his place,” Mr. Allen wrote in his 2001 memoir of Vietnam, “None So Blind.” “I decided to respond candidly.”

“Stop the buildup of American forces,” he said he told Mr. McNamara. “Halt the bombing of the North, and negotiate a cease-fire with Hanoi.”

After that moment of truth, Mr. McNamara told his aides to begin compiling a top-secret history of the war — later known as the Pentagon Papers — and he began asking himself what the United States was doing in Vietnam. Many Americans were asking the same, giving rise to a growing antiwar movement that even Mr. McNamara’s own son participated in as a student protester at Stanford.

On Sept. 19, 1966, Mr. McNamara telephoned Johnson.

“I myself am more and more convinced that we ought definitely to plan on termination of bombing in the North,” Mr. McNamara said, according to White House tapes.

He also suggested establishing a ceiling on the number of troops to be sent to Vietnam. “I don’t think we ought to just look ahead to the future and say we’re going to go higher and higher and higher and higher — 600,000; 700,000; whatever it takes.”

The president’s only response was an unintelligible grunt.

Departure and Guilt

The turning point came on May 19, 1967, when Mr. McNamara sent a long and carefully argued paper to Johnson, urging him to negotiate a peace rather than escalate the war.

The war, the paper began, “is becoming increasingly unpopular as it escalates — causing more American casualties, more fear of its growing into a wider war, more privation of the domestic sector, and more distress at the amount of suffering being visited on the noncombatants in Vietnam, South and North.”

“Most Americans,” Mr. McNamara continued, “are convinced that somehow we should not have gotten this deeply in. All want the war ended and expect their president to end it. Successfully. Or else.”

That was the last straw for Johnson, who came to believe that Mr. McNamara was secretly plotting to help Robert Kennedy, then a Democratic senator from New York, run on a peace ticket in the 1968 election. The president announced on Nov. 29, 1967, that Mr. McNamara would give up his defense post to run the World Bank. Mr. McNamara left the Pentagon two months later, never comprehending, in his words, “whether I quit or was fired.” It was clearly the latter.

Mr. McNamara had sought to transform the armed services. But his often aloof and occasionally arrogant conduct left him with few allies inside the Pentagon when the war began to go wrong. At a going-away luncheon given by Secretary of State Dean Rusk, Mr. McNamara wept as he spoke of the futility of the air war in Vietnam. Many of his colleagues were appalled as he condemned the bombing, aghast at the weight of his guilt.

He had thought for a long time that the United States could not win the war. In retirement, he listed reasons: a failure to understand the enemy, a failure to see the limits of high-tech weapons, a failure to tell the truth to the American people and a failure to grasp the nature of the threat of communism.

“What went wrong was a basic misunderstanding or misevaluation of the threat to our security represented by the North Vietnamese,” he said in his Berkeley oral history. “It led President Eisenhower in 1954 to say that if Vietnam were lost, or if Laos and Vietnam were lost, the dominoes would fall.”

He continued, “I am certain we exaggerated the threat.”

“We didn’t know our opposition,” he said. “We didn’t understand the Chinese; we didn’t understand the Vietnamese, particularly the North Vietnamese. So the first lesson is know your opponents. I want to suggest to you that we don’t know our potential opponents today.”

An Analytical Mind

Robert Strange McNamara — Strange was his mother’s maiden name — was born June 9, 1916, in San Francisco to Robert and Clara Nell McNamara. His father, the son of Irish immigrants, managed a wholesale shoe company.

“My earliest memory is of a city exploding with joy,” he said in “The Fog of War.” It was Nov. 11, 1918 — the end of World War I. He remembered the tops of the streetcars crowded with people cheering and kissing.

In 1937, Mr. McNamara graduated with honors in economics from the University of California, Berkeley, where he also studied philosophy. After two years at Harvard Business School, he spent a year with Price, Waterhouse & Company, the accounting firm. He returned to Harvard in 1940 as an assistant professor of business administration.

That year, he married his college sweetheart, Margaret Craig. She created Reading Is Fundamental, a literacy program for poor children, while he was at the Pentagon. By the time she died in 1981, the program served three million children.

Mr. McNamara and his second wife, the former Diana Masieri Byfield, were married in 2004 in San Francisco.

Besides his wife, Mr. McNamara is survived by his son, Robert Craig, of Winters, Calif.; two daughters, Margaret Elizabeth Pastor and Kathleen McNamara, both of Washington, and six grandchildren.

When World War II came, Mr. McNamara taught young air officers the statistical methods he had learned at Harvard, with the aim of orchestrating the air war in Europe by determining how many planes could fly each day in every theater. He served in England, then India, and held the rank of lieutenant colonel at war’s end in 1945.

“After the war, my wife and I both came down with polio, if you can imagine, infantile paralysis,” Mr. McNamara remembered in his memoir. “My case was relatively light; I was out of the hospital in a couple of months. But she was in the hospital for nine months, and they thought she’d never lift an arm or a leg off the bed again.”

Unable to pay the hospital bills on a Harvard salary, he accepted a job offer from the Ford Motor Company.

He and nine other air-war statisticians, none older than 30, were hired by Henry Ford IIto reorganize a mismanaged company.

“He wanted some individuals who he could feel were his men, if you will, because the company was staffed with old-line executives who had been associated with his father and grandfather,” Mr. McNamara recalled.

The company lost $85 million in the first eight months after Mr. McNamara’s arrival, the equivalent of about $925 million adjusted for inflation today. But Mr. McNamara and his young team turned Ford around. He rose swiftly — comptroller, general manager of the Ford division, vice president for all car and truck divisions.

In November 1960, one day after Kennedy’s election, Mr. McNamara was named president of the company, the No. 2 position under Mr. Ford, who was chairman and chief executive. Five weeks later, Kennedy asked him to run the Pentagon.

The World Bank Years

Mr. McNamara’s time at the Pentagon came close to breaking his spirit. But he immediately followed that ordeal with 13 years as president of the World Bank. He set out to expand the bank’s power and to attack global poverty. He succeeded in part, but with unintended consequences.

The industrialized nations created the bank at the end of World War II to help rebuild Western Europe, but it later expanded its membership and shifted its focus to lending in the third world to increase economic growth and forestall war. In 1973 Mr. McNamara dedicated himself to the reduction of what he called “absolute poverty — utter degradation” in Africa, Asia, and Latin America.

As he had done at the Pentagon and Ford, Mr. McNamara sought to remake the bank. When he arrived on April 1, 1968, the bank was lending about $1 billion a year. That figure grew until it stood at $12 billion when he left in 1981. By that time the bank oversaw some 1,600 projects valued at $100 billion in 100 nations, includinghydroelectric dams, superhighways and steel factories.

The ecological effects of these developments, however, had not been taken into account. In some cases, corruption in the governments that the bank sought to help undid its good intentions. Many poor nations, overwhelmed by their debts to the bank, were not able to repay loans.

The costs of Mr. McNamara’s work thus sometimes outweighed the benefits, and that led to a concerted political attack on the bank itself during the 1980s.

Mr. McNamara saw some of these problems as they developed and shifted the emphasis of the bank’s lending toward smaller projects — irrigation, seeds and fertilizer, paving farm-to-market roads. But progress was often hard to measure. At the end of his tenure, the bank estimated that the world’s poorest numbered 800 million, an increase of 200 million over the decade.

Public Contrition

Mr. McNamara left the bank when he turned 65, after his wife died, and for a time he tried to unwind and get away, taking a 140-mile hike up to the 18,000-foot level of Mount Everest. But within two years, he began to speak out against the nuclear arms race. In 1995, 14 years after leaving public life, he published his denunciation of the Vietnam War and his role in it, “In Retrospect: The Tragedy and Lessons of Vietnam” (Times Books/Random House), for which he was denounced in turn.

Unlike any other secretary of defense, Mr. McNamara struggled in public with the morality of war and the uses of American power.

“We are the strongest nation in the world today,” Mr. McNamara said in “The Fog of War,” released at the time of the 2003 invasion of Iraq. “I do not believe that we should ever apply that economic, political, and military power unilaterally. If we had followed that rule in Vietnam, we wouldn’t have been there. None of our allies supported us. Not Japan, not Germany, not Britain or France. If we can’t persuade nations with comparable values of the merit of our cause, we’d better re-examine our reasoning.”

“War is so complex it’s beyond the ability of the human mind to comprehend,” he concluded. “Our judgment, our understanding, are not adequate. And we kill people unnecessarily.”

Robert Mcnamara