Dean Rusk, who as Secretary of State for President Kennedy and President Johnson became a stubborn if much-criticized defender of the American involvement in the Vietnam War, died Tuesday at his home in Athens, Ga., with his family at his side. He was 85.

The cause of death was congestive heart failure, according to a spokesman for the University of Georgia.

Mr. Rusk, a former president of the Rockefeller Foundation, was Secretary of State all through the Kennedy and Johnson Administrations, from 1961 to 1969. He returned to his native Georgia in 1970 and taught international law at the University of Georgia in Athens until he retired in 1984.

Other than Cordell Hull, who was Secretary of State for President Franklin D. Roosevelt in the 1930's and 40's, Mr. Rusk held the job longer than any predecessor -- a total of eight years. He felt driven to defend and justify the American involvement on the side of South Vietnam, when many in the public and in Congress turned against it in the later years of the Johnson Administration.

Some of the more memorable moments in Washington in the 1960's included Mr. Rusk's stout defense of the Vietnam policy in front of a Senate Foreign Relations Committee headed by J. W. Fulbright, who had broken with the policy.

As Secretary, Mr. Rusk played almost two different roles. Under President Kennedy, his position was much less defined, with the President often taking advice from other officials and not paying that much attention to Mr. Rusk. But Mr. Johnson, himself an outsider in the Kennedy Administration, relied increasingly on the advice of his fellow Southerner.

Mr. Rusk's belief, which he never tired of stating, was that the United States had a commitment to South Vietnam that it could not break without risking a larger war with China or Russia.

He declared again and again that "as far as the United States is concerned, we have a commitment to South Vietnam -- and we shall meet it."

Mr. Rusk was also deeply involved in the hot and cold American relations with the Soviet Union. He helped engineer the first arms control accords with Moscow. And he was President Kennedy's adviser when the Soviet Union, in Mr. Rusk's word, "blinked" during the Cuban missile crisis. Praised by Johnson, Reviled by Protesters

In the Johnson years, Mr. Rusk was praised by a grateful President, who said: "He's got courage. A Georgia cracker. When you're going in with the marines, he's the kind you want at your side."

But Mr. Rusk came to be reviled by anti-war protesters, who mobbed his appearances and shouted insults and slogans like "Murderer!" and "Stop the bombing!" By the end of his service, he rarely spoke to groups outside Washington for fear of provoking such protests.

Born in rural Cherokee County, Georgia, Mr. Rusk never lost his Southern accent, gracious demeanor and fondness for a good story.

During the increasingly bitter discussions about Vietnam in the mid-1960's, he was often asked by critics at public meetings why the United States was supporting a Government in South Vietnam that was not backed by its own peasants. To this he would answer: "I don't remember waking up in Cherokee County, pounding my chest, and saying, 'What has Woodrow Wilson done for me today.' " His anecdote usually led his audience to chuckle and won him time to continue his arguments for the need to persevere in Vietnam.

His background clearly had an impact on his days in Washington. In the 1930's, he was a Rhodes Scholar and used to recall the debates in Oxford over whether Britain should stop Hitler in his tracks when he was beginning to seize territory. The overwhelming sentiment, he remarked, was isolationist in Britain, and as a result only a few years later, Britain was forced to war to defends its island.

The moral he drew was that if America stopped the North Vietnamese now, it would not have to fight a larger war later.

Then and in later decades, his work as Secretary drew mixed reviews from historians, biographers and foreign-policy experts.

James Chace of the Carnegie Endowment for International Peace wrote in 1988, in The New York Times Book Review, that Mr. Rusk was "a good man -- loyal, intelligent and self-sacrificing -- who was marked by a fatal lack of imagination and who came to bear the onus for perhaps the most tragic failure of American foreign policy in this century, the waging of the Vietnam War." Views on Vietnam Evolved to Firmness

Mr. Rusk's views on Vietnam changed somewhat, but by the mid 1960's he believed firmly that that the United States had to be very forceful in confronting aggression by the army of Communist North Vietnam in South Vietnam, whose Government was pro-Western. Otherwise, he said, the United States would be perceived as weak and would invite further Communist aggression, which could touch off nuclear wars.

While Mr. Rusk was Secretary, the American role grew until almost 550,000 United States troops were in South Vietnam in 1969, allied with the shaky South Vietnamese Army against the North Vietnamese invaders and the Communist Vietcong insurgents. At the same time, United States bombers were pounding North Vietnam.

But he failed to foresee that the Vietnamese Communist forces would keep on fighting despite the vast United States intervention.

Eventually, the Nixon Administration progressively withdrew United States forces from South Vietnam. The last troops left in 1973, after more than 50,000 Americans had died in the war. The Communists then completed their takeover of the South, uniting it with the North.

The Vietnam War later was generally thought to be "the first war which the United States unequivocally lost," as the Yale historian Paul Kennedy wrote in his 1988 book "The Rise and Fall of the Great Powers."

Yet Mr. Rusk did not admit any American errors when he was asked, "What went wrong in Vietnam?" at a news conference shortly before he left office with the end of the Johnson Administration in 1969. He quickly responded, "What went wrong was a persistent and determined attempt by the authorities in Hanoi to take over South Vietnam by force."

In interviews in later years and in his memoirs, however, he said he had made two mistakes about the Vietnam War: underestimating the tenacity of the North Vietnamese and overestimating the patience of the American people.

Mr. Rusk came to spend most of his time as Secretary dealing with Vietnam, and his role in some other notable foreign-policy spheres was less publicized and less crucial. He had only a minor hand in the unsuccessful Bay of Pigs invasion of Cuba in 1961, early in the Kennedy Administration, by Cuban exiles trained and equipped by the Central Intelligence Agency. His role was partly passive during the Cuban missile crisis in late 1962.

In good times and bad, Mr. Rusk generally radiated self-confidence and solidity as Secretary. He stood 6 feet 1 inch tall and weighed 200 pounds, and even near the end of his time in office, he remained "dogged, durable, unfailingly courteous and considerate," as he was described then by the chief Washington correspondent of The New York Times, Max Frankel.

But Mr. Rusk's son Richard wrote, in a commentary included in Dean Rusk's memoirs, that his father began having chronic stomach pain in the months preceding the buildup in Vietnam in 1965. The son wrote that the pain was "never diagnosed and persisted into old age."

Before he became Secretary, Dean Rusk gained wide knowledge and experience in Asian matters and foreign affairs in general. He was a student at Oxford as a Rhodes Scholar in his early 20's and also studied in Germany.

Later he was an Army staff officer in the China-Burma-India theater during World War II and a senior State Department official during part of the Korean War. He took a particular interest in Asia as head of the Ford Foundation, which is based in New York, from 1952 to 1960.

In his memoirs, "As I Saw It" (Norton, 1990), Mr. Rusk said a major lesson he had learned from World War II "was that if aggression is allowed to gather momentum, it can continue to build and lead to general war."

Tracing what he saw as the evolution in Mr. Rusk's thinking on Vietnam, William Bundy, Assistant Secretary of State for Far Eastern Affairs in 1965, wrote later that by that summer, "Dean Rusk, skeptical in the fall of 1961 and uncertain in January of 1965 that South Vietnam had the political cohesion to be worth going all out for, had now concluded that to walk away from an American commitment, to temporize in our actions to meet it, to do anything short of all we possibly could, would pose the gravest possible danger to world peace." 'Widely Regarded As the Chief Hawk'

As the war went on, opposition to the stepped-up American involvement grew more intense in the United States. In that period Thomas J. Schoenbaum, a colleague of Mr. Rusk's on the University of Georgia faculty, wrote in "Waging Peace and War: Dean Rusk in the Truman, Kennedy and Johnson Years" (Simon & Schuster, 1988), the Secretary "made himself the rock against which crashed the successive waves of dissent" and was "widely regarded as the chief hawk in the aerie of Vietnam advisers."

Critics also charged that Johnson Administration statements about the war could not be trusted, that there was "a credibility gap." Professor Schoenbaum wrote that in that era, in answering journalists' questions, Mr. Rusk "was often vague and evasive." "Although he denies having lied to the press," Professor Schoenbaum continued, "his half-truths and incomplete answers were frequently misleading."

Meanwhile, Mr. Rusk wrote in his book, the Johnson Administration made numerous "efforts to find a formula for peace with Hanoi" because "we wanted a negotiated end to the war." He wrote that he was told by Mr. Johnson soon after he became President, "Your mission is to bring about a peaceful settlement of this situation at the earliest possible moment."

But Mr. Rusk failed in that mission, despite halts in the United States bombing of North Vietnam that were meant to promote it.

More broadly, the Vietnam policies with which Mr. Rusk was so closely identified as Secretary came to be widely regarded as particularly dismal failures, and more than a few historians and other analysts came to hold critical views of his work on Vietnam.

Professor Warren I. Cohen, director of the Asian Studies Center at Michigan State University, wrote in his biography, "Dean Rusk" (Cooper Square Publishers, 1980), that Mr. Rusk had not grasped the extent to which "the 'aggression' of a national liberation movement is much more complicated morally and much harder to cope with politically or militarily" than the aggression of a great power.

The outcry against the war spread across America, spurred partly by the Communists' broad Tet offensive early in 1968, even though the attack was a military failure.

In March of that year Mr. Rusk, in another secret memorandum, argued against authorizing a further enlargement of the American ground force in Vietnam and advocated a cutback in the American bombing of North Vietnam. A shaken Mr. Johnson later that month decreed a sweeping cutback in the bombing. He also refrained from additional big troop buildups.

The partial bombing halt was followed by the inconclusive beginning of negotiations between the United States and North Vietnam in Paris, and Mr. Johnson later halted all bombing of the North. Formula in Office: Endure and Survive

In carrying out his manifold duties as Secretary, Mr. Rusk worked hard and persistently, traveling very widely. Despite grave East-West tensions, he succeeded in developing a fairly good working relationship with the Soviet Foreign Minister, Andrei A. Gromyko.

Nonetheless, others found fault with his overall performance as Secretary. Appraising it near its end, Mr. Frankel wrote in The New York Times: "His formula, under Presidents Kennedy and Johnson, for self and country, was to endure and to survive, to keep playing a mediocre hand rather than risk all for a better one, and to stay around for greater achievement another day. He never stood for boldness or imaginative innovation."

President Kennedy originally chose Mr. Rusk as his Secretary of State, rather than some better-known figure, because as the historian and former Kennedy White House aide Arthur M. Schlesinger Jr. wrote in his book "A Thousand Days" (Houghton Mifflin, 1965), Mr. Kennedy "wanted to be his own Secretary of State."

He thought that Mr. Rusk, who had worked well in large organizations, was the right man to carry out Kennedy foreign policies. Mr. Schlesinger reported that the President came to be impressed by Mr. Rusk's "capacity to define" but depressed by his "reluctance to decide."

Mr. Rusk himself later concluded that he acted too deferentially when the fledgling Kennedy Administration was pondering plans for what became the 1961 Bay of Pigs invasion of Cuba. In his book he wrote of his reaction when he heard about the plans: "Having been in the China-Burma-India theater in World War II, I knew that this thin brigade of Cuban exiles did not stand a snowball's chance in hell of success. I didn't relay this military judgment to President Kennedy because I was no longer in the military."

He reported that he privately expressed to Mr. Kennedy his opposition on non-military grounds.

But he wrote that in retrospect, "in replaying and rethinking everything, I wished that I had pushed my reticence aside and organized resistance to the invasion" in the Administration.

The invasion was quickly crushed by Fidel Castro's forces. Mr. Rusk wrote that out of loyalty to Mr. Kennedy, he did not let it be known at the time that he had privately opposed it. A Reserved Role In Missile Crisis

Mr. Rusk also chose to play a somewhat passive role at the outset of the Cuban missile crisis of late 1962, which was one of the tautest cold war standoffs between the United States and the Soviet Union. It arose after the Kennedy Administration learned from aerial photographs, taken on Oct. 14, that Soviet missile-launching installations were being constructed secretly in Cuba.

Mr. Rusk held back from the debate among the President's advisers. In his book, he wrote that "as Secretary of State, I felt that I should hold myself in reserve and hear all points of view before giving my recommendation to the President."

After five days of secret Administration deliberations, he wrote, he and Defense Secretary Robert S. McNamara recommended to the President that he order a partial naval blockade of Cuba.

Mr. Kennedy did so, and also threatened to retaliate militarily against the Soviets if a missile were to be launched from Cuba. Not long afterward, the tension eased somewhat when, as Mr. Rusk recalled in his book, several Soviet ships that were suspected of carrying missiles for delivery in Cuba "stopped dead in the water, then turned around and headed back to the Black Sea."

He added, "When we heard this news, I said to my colleagues, 'We are eyeball to eyeball, and the other fellow just blinked.' "

The crisis ended on Oct. 28 when Nikita S. Khrushchev declared in Moscow that the missiles would be pulled out of Cuba under United Nations supervision.

Mr. Rusk went on to help bring into being the partial Nuclear Test-Ban Treaty that was signed by the United States, the Soviet Union and Britain in 1963. Other nations later adhered to it.

After Mr. Rusk stepped down as Secretary in 1969, it took a while for him to find a permanent job. Professor Schoenbaum wrote, "Not only did he have difficulty finding work, but many people, including many former friends, despised him."

Mr. Rusk went on to become Sibley Professor of International Law at the University of Georgia at Athens, in Clarke County, 45 miles southeast of his native Cherokee County. A Native Georgian And Star Student

David Dean Rusk was born in Cherokee County, Ga., on Feb. 9, 1909, the son of Robert Hugh Rusk, who had been obliged by a voice ailment to quit the Presbyterian ministry, and Elizabeth Clotfelter Rusk.

Robert Hugh Rusk went on to farm and teach school for some years in the county, and as a small boy there, his son ran around in underwear made from flour sacks. Then the family moved to Atlanta, where the father became a mail carrier, and young Dean became a star high school student. He went on to work his way through Davidson College, in Davidson, N.C., where he was elected to Phi Beta Kappa and graduated in 1931.

Going on to St. John's College of Oxford University as a Rhodes Scholar, he earned an M.A. degree in philosophy, politics and economics in 1934.

He then worked at Mills College in Oakland, Calif., from 1934 to 1940, teaching Government and International Relations and serving as dean of the faculty while also studying law at the University of California at Berkeley.

An Army Reserve officer, Mr. Rusk was called to active duty in 1940 and remained in uniform until 1946, rising to the rank of colonel. He saw service with the military intelligence branch of the Army General Staff, as a staff officer under Gen. Joseph W. Stilwell in the China-Burma-India theater and back in Washington, in the Operations Division of the War Department General Staff

He joined the State Department in 1946 as assistant chief of the division of international security affairs and went on to serve as a Special Assistant to the Secretary of War in 1946 and 1947 and as head of the State Department's United Nations desk from 1947 to 1949.

From that year until 1951, he was successively an Assistant Secretary of State, a Deputy Under Secretary and Assistant Secretary for Far Eastern Affairs.

As president of the Rockefeller Foundation, he wrote in his book, he felt that "the third world, where two-thirds of the world's population live, was a time bomb for the entire human race," adding, "We channeled more funding to the developing countries in Africa, Latin America and Asia."

Mr. Rusk was married to Virginia Foisie in 1937.

He is survived by his wife; two sons, David of Washington and Richard of Bishop, Ga.; a daughter, Peggy Smith of Stafford, Va., and six grandchildren.

A funeral service is planned at Athens First Presbyterian Church.