In the five months since his biography of Cold War diplomat George Kennan came out, John Lewis Gaddis has been toasted as a master historian, and roasted as a conservative who minimized Kennan's liberal tendencies.

Now he's won the Pulitzer Prize _ and he'd like readers to just take in the story.

"I didn't have any particular agenda in mind," Gaddis says. "My hope is, and I think it has been borne out, that people would respond to the book on its own merits."

Gaddis was widely acknowledged as the obvious choice to tell the story of Kennan's life. A published author for more than 40 years, he has been called the dean of Cold War thinkers by Harvard historian Priscilla McMillan. Evan Thomas, whose book "The Wise Men" includes a chapter on Kennan, says Gaddis is a "master" who makes an "honest effort to cut through cant and ideology." In 2005, Gaddis received a National Humanities Medal for "his incisive examination" of the epic conflict between the capitalist West and communist East.

But while Gaddis is an insider _ a popular teacher at Yale University, winner of numerous awards, a guest at the White House _ he's an outsider to many colleagues in New Haven and elsewhere. He has kind words for Ronald Reagan and became close enough to George W. Bush to advise him on his second inaugural address and on his memoir "Decision Points," which Gaddis includes in a class he teaches on biography. Henry Kissinger is a supporter of the "Studies in Grand Strategy" course Gaddis helps teach and wrote a highly favorable review of the Kennan book for The New York Times.

So while the Pulitzer board praised "George F. Kennan" as "an engaging portrait" of the quintessential Cold War diplomat and the times he lived in _ and the National Book Critics Circle cited Gaddis' "profound understanding of U.S. foreign policy in the 20th century" in awarding him its biography prize _ Gaddis has been criticized for omitting or disregarding some of Kennan's more liberal opinions. Eric Alterman of the liberal weekly The Nation labeled the book "Strategies of Disparagement."

"It's fair to say I am more conservative than most of the Yale faculty," Gaddis observes during a recent interview at his Yale office, where pictures of him with Bush and Kissinger hang on the walls. "I'm used to it, but certainly it's not always a popular position. Universities are rather intolerant places and there are orthodoxies within universities. Political correctness is not what it once was, but it does still exist."

Kennan himself had differences with his biographer. Gaddis thought Reagan was a visionary who ended the Cold War and the nuclear arms race; Kennan worried the president would blow us all to Kingdom Come. Gaddis supported the Iraq war, Kennan opposed it. Kennan, a born brooder, wondered whether Gaddis was the right man.

"I think he got a little nervous at times because I was a little more to the right of him on the current political issues than he was," Gaddis says. "I was more sympathetic to Ronald Reagan, for example, and later to George W. Bush, for sure. But we never got to the point where he said, `Because of your politics you are no longer qualified to write the biography.' He never did, and never came close to it."

And Kennan had a long time to second-guess his choice. Gaddis first met Kennan in the mid-1970s and felt enough of a rapport to send some pages from an upcoming book about the Cold War. They became friendly and agreed in the early `80s that Gaddis write his story. Gaddis would be granted full access to Kennan, his family and friends and to Kennan's papers. Kennan, in his 70s at the time, sought no editorial control. But he did ask that the book not be released until after his death.

Kennan lived to 101.

"Poor John Gaddis has seen his undertaking being put off for years while he waits for me to make way for it," Kennan, who died in 2005, wrote in his diary.

Gaddis says his goal was to present his subject fully and fairly, with flaws and virtues accounted. Kennan had much to offer on each side. He was a tireless seeker of knowledge and a first-rate prose stylist who won two Pulitzer Prizes. His influence far outweighed his rank; Kennan was a member of the foreign service who never held a high-level position.

But as a member of the diplomatic corps in Moscow, his intimate knowledge of the Soviet present and the Russian past gave him near-prophetic powers. He anticipated that Marxism was just a phase in the country's history. He was an architect of the Marshall Plan, which helped revive the economies of Western Europe after World War II and helped undermine Stalin's belief that the West would turn against itself. He believed early on that that the Soviet Union and China would quarrel despite a shared belief in Communism.

Kennan was also the most human of visionaries. He had several extra-marital affairs. He was highly sensitive and impatient and once wrote in his diary that he dreaded "any occupation that implies any sort of association with, and adjustment to, other people." His call in 1946-47 for "containment" of the Soviet Union was a victory for anti-Communists who doubted that the U.S. could remain on good terms with its World War II ally. Yet Kennan found himself to the left of Washington for decades after, whether on Vietnam or the nuclear arms race.

"He came up with the most influential post-World War II strategy and within a year of having done so began to repudiate it," Gaddis says. "Kennan was one of those people who felt his ideas were not working unless he was personally putting them into effect."

"George F. Kennan" has an ironic subtitle: "An American Life." Kennan lived abroad for long periods of time and seemed out of place when he returned. He disdained American culture and had limited taste for electoral or office politics. His sensibility was not of a campaigner, but of an artist. He wrote poetry and played guitars. His great dream was not to become president, but write a biography of Chekhov.

"Your understanding of the subject of any biography is broadened and deepened and complicated by any act of biography," Gaddis says. "I've always seen the word `critical' as having both a positive and negative meaning. To be a critic is to praise and to complain. But I still came out of this book extremely impressed by George and with an increased admiration and respect for him."

Over the past 40 years, the 71-year-old Gaddis had written several books on Cold War policy. But prior to "George Kennan," he had never written about an individual life.

As he researched and wrote Kennan's story, the historian decided to educate himself by offering a class in biography _ not a lecture, but one centered on discussion.

During a recent class at Yale, the assigned book was Robert Caro's "Means of Ascent," the second and most controversial of Caro's series on Lyndon Johnson. "Means of Ascent" portrayed a politician so boorish and unscrupulous that former LBJ aides accused Caro of trying to destroy Johnson's reputation.

But Gaddis, a low-key instructor with an even, probing style, notes that the book is prefaced by Caro's vivid portrait of one of Johnson's noblest moments _ the 1965 civil rights speech he gave as president, when he brought tears to the eyes of the Rev. Martin Luther King Jr. and others by invoking the title of the protest movement's anthem, "We Shall Overcome."

So what kind of man was LBJ, Gaddis asks 20 undergraduates seated around a conference table? And what kind of system did he work in? Could the achievements of his presidency have been possible without the misdeeds of the Senate race years earlier? Is life ever without compromise? The students reach no conclusion, and Gaddis wasn't expecting one. After the class, he explains that of all the lessons he's taken in as a biographer, none is more important than leaving some questions unanswered.

"A really good biography does not have to resolve all issues," Gaddis says. "It can leave contradictions there. It can just say these contradictions were there and were important and the subject of the biography never completely resolved them." The subject of the biography himself was torn by contradictions. And this is certainly true of George Kennan."

Some critics allege Gaddis turned against his subject. In The New York Review of Books, Frank Costigliola's analysis was titled "Is This George Kennan?" He called the book "monumental and absorbing" but worried about the "perspective and balance," noting Gaddis never mentioned that in 1968 Kennan endorsed an anti-war Democrat, Eugene McCarthy, for president.

"The biography suffers from this neglect," Costigliola writes.

Gaddis acknowledges that he could have included Kennan's support for McCarthy, but said he found it more important to write about Kennan's televised Senate testimony in 1966, when he called the Vietnam commitment "unsound" and chastised the United States for acting like "an elephant frightened by a mouse." The book, Gaddis emphasizes, does not fit any political category.

Conservatives think highly of Gaddis, and liberals disapprove, but he says he's a registered independent who has voted for Democrats and Republicans, from Jimmy Carter in 1980 and Bill Clinton in the 1990s to Ronald Reagan in 1984 and George W. Bush in 2000 and 2004. (He voted for Barack Obama in 2008, but remains undecided for this year).

During his seminar, Gaddis spoke warmly about the New Deal and the civil rights movement. In his books, he has expressed great skepticism about the Vietnam War, dismay at the "outright deception" of Kissinger and Richard Nixon and disappointments about the Iraq war. He says during his interview that it "was a great failing of the Bush administration" not to know more about the culture of Iraq before invading and for relying on bad intelligence.

"These were big mistakes," he says.

Criticism of Bush actually helped lead to Gaddis' meeting the president. In 2004, he published a brief book, "Surprise, Security, and the American Experience," which defended the right to "preemptive war," but also faulted the administration's "shock and awe" military campaign. Condoleezza Rice, then national security advisor and an old acquaintance of Gaddis', asked the historian to meet with her staff.

According to Rice's memoir, "No Higher Honor," Gaddis encouraged her to take a more diplomatic approach to the country's allies. As Rice would acknowledge, "repair work" was needed. When they were done, she surprised Gaddis by bringing him to the Oval Office to meet the president.

"I was thinking it would be a photo op," Gaddis says. "But he had read the book. He underlined it. He had taken notes on it. ... We kind of hit it off at that point."

Some of Gaddis' former students have gone on to careers in Washington. Chris Michel became a White House speechwriter under Bush and later worked with Bush on "Decision Points." Keith Urbahn was an aide to former Defense Secretary Donald Rumsfeld who now runs Javelin Group LLC, a communications and book firm based in Washington.

Urbahn recalls taking Gaddis' "Grand Strategy" class at the height of the Iraq war's unpopularity: "He saw his role as raising larger questions that you had to grapple with. He cultivated a generation of students to think in terms of practical decision making. He didn't have this air of knowing sophistication that I felt with a lot of other professors."

"I don't think of him as a conservative," Michel adds. "But anyone who has nice things to say about George W. Bush is going to stand out among the Yale faculty."

Gaddis was born in Cotulla, Texas in 1941. The community was small, and personal. During his biography class, the historian asked his students to imagine a man on a tractor, age 25, working in a Texas field in the 1920s. It's hot, the land is flat and dusty. The man spots a Model-T pulling up and a young stranger getting out, dressed in a blue serge suit. He climbs through a barbed wire fence and approaches.

"Hi, I'm Lyndon Johnson and I'm the new high school teacher."

"The hell you are," is the reply.

Adds Gaddis: "The man on the tractor was my father."

He credits teachers in high school with inspiring him to become a history major. Gaddis was an undergraduate at the University of Texas at Austin and remained there for his master's and Ph.D. He specialized in the Cold War in part out of "ambition" and out of awareness that it was a relatively new field, a story just beginning to be told.

As an author, he established himself with his first book, "The United States and the Origins of the Cold War," published in 1972. At the time, Cold War scholarship had been shaped by such New Left historians as William Appleman Williams, who had written that economic reasons, especially the need for markets overseas, were a principle force behind U.S. foreign policy. Gaddis countered that capitalism was just one part of a conflict that included domestic politics, Marxist ideology and the personalities of Stalin, Mao and other leaders.

"I found some of the New Left views valuable: the emphasis on the economic dimensions of foreign policy, and, flowing from that, their insistence that there'd been more continuity in it throughout the 20th century than older historians had perceived," Gaddis says. "What I did not find convincing was their argument that the need to export drove the Americans into an aggressive foreign policy, and that had it not been for this, the Russians would have continued to be allies. The New Left's greatest weakness was always its lack of interest in, or curiosity about, the USSR."

Gaddis' scholarship has been a story of revision. In the 1970s, historians had no access to Soviet or Chinese documents, and the world itself seemed deadlocked between rival superpowers. Within 20 years, the Cold War was over and the Soviet Union had broken up. Secrets once vital were now expendable; Gaddis and others could finally learn what Stalin and other Eastern bloc leaders were thinking.

In "We Now Know," published in 1997, Gaddis revisits such Cold War topics as why North Korea invaded South Korea (Stalin encouraged it, assuming the United States would not respond), how frightened the Soviets might have been by the atom bomb (more than they let on) and the assumption that Stalin and others valued survival above all and never really thought Marxism would defeat and destroy capitalism.

"That was the prevailing wisdom, which I certainly bought into, that the ideological rhetoric of the Chinese, Russians and East Europeans was window dressing," Gaddis says. "But as I began to go into the documents, I discovered that the language was the same in the secret meetings as it was in the public pronouncements. They really believed this stuff."