The Third World War began in November 1976, with a letter from Lord Longford in London to Sir John Hackett in his 16th-century millhouse home in the rolling green hills of Gloucestershire 90 miles southwest of here.

Lord Longford, chairman of the Sidwick and Jackson Publishing House, knew he had contacted the right man to stage and chronicle World War III. Sir John, as Lieutenant General Sir John Winthrop Hackett, had commanded Britian's Army of the Rhine and NATO's northern army group before retiring from the army in 1968. Hackett had already won World War III once, as commander of the Russian army in a massive NATO war game in 1967, when he pushed the hapless Allies back to the Rhine in just three days.

Lord Longford also knew there was still more to this most unusual general - who had earned honors at Oxford in both English classics and modern history, who was fluent in German, French and Italian and read both Latin and Greek, who wrote a doctoral thesis on the crusades for still another degree while serving as a young army officer in the Middly east, who after his military retirement had been chancellor of King's College of the University of London and president of both the United Kingdom Classics Association and the English Association, and who was now visitng professor of classics at King's College.

Besides, Hackett had already written one book for Lord Longford, which Sir John still believes "was a much better book" than "The Third World War." "But it was a flop." That first book, "I Was a Stranger," published in the United States last year, was a memoir of the three months Hackett spent at the end of World War II hidden by a Dutch family behind enemy lines in the Netherlands after he was badly wounded at Arnheim.

"We were just 50 yards from a German MP post, right in the eye of the hurricane," Hackett remembered. "But like the eye of the hurricane, there was a special peace in this household. I read Shakespeare and the authorized version of the Bible straight through. The book did apparently have quite and effect on those who read it. Not a week goes by without my receiving letters about it."

While Jimmy Carter and Leonid Brezhnev are discussing the safety of civilization at the Vienna summit, "The Third World War" is near the top of American best-seller lists - including Washington's. In fact, the book's worldwide success has been so great, Hackett noted with a wry smile, that there is talk of giving "I Was a Stranger" another push in American bookstores.

Wry smiles are a frequent occurence just below the neat gray mustache on Sir John Hackett's face. Now 68, he is a small balding, delicately boned man who looks much more like the full-time scholar he has been for a decade that the Cold War general he was before that. He seemed quite at home in the antique and book-filled factulty apartment at the Imperial College of Science and Technology here, where he and his wife stayed recently while he delivered the college's third annual Jubilee Lecture on real-life prospects of a third world war.

Hackett smiled most when the conversation strayed from the troops, tanks and nuclear exchanges of his fictional third world war to the schoolarly loves of his life. He recalled with evident pleasure how he slipped away from his comrades in arms on his offduty time while stationed in Egypt and Palestine in the 1930s to explore the ruins of crusader castles and read medieval accounts of the crusades in the Jesuit university in Beirut. The American University there, he noted with a wry smile, "was of no use to me because they had nothing original, only books about books about books."

He also received and offer then to teach at Oxford University. "I was tempted." he said, "but I finally deceided to keep on soldiering for a while."

His soldiering eventually took Hackett up Britain's militaty career ladder to the penultimate step from the top. It may have been only his sometimes serious, sometimes impish nonconformist nature that kept him from the very top. It was this same nonconformity that later led him, in the early 1970s as chancellor of King's College, to join his students in a protest march "for higher student grants" and to argue publicly that, if marijuana were legalized, the tax revenue could be used for university research. Needless to say, the wry chancellor was popular with his students, who never rebelled during that period of student unrest, but a bit alarming to the rest of the establishment.

"When people ask why I wasn't chief of staff of this country," Hackett explained, the smile palying under his mustache again, "I say that not only could I not have stood it, but neither could have anybody else."

His boldest non-conformist act was the letter he wrote to the Times of London in 1968 declaring that NATO was no longer in any shape to stand up to the Russians. The problem was that Hackett was still commander of the Army of Rhine and NATO's northern army group at the time. Before sending the letter, Hackett went to see Britain's defense secretary, burly and bluff Dennis Healey (who most recently was chancellor of the Exchequer in the Labour Party government vored out of office last month).

"I told him," Hackett recalled, "that I wanted to write a letter to The Times, Le Monde, Die Welt and some other newspapers telling NATO to pull its socks up."

Healey gruffly reminded Hackett that he was a British general and would have to show the letter first to Healey for clearance, which he probably would not five him.

"So I asked him," Hackett said, "wasn't I also a NATO general? And he answered, "Yes, but you are a British general part of the time."

"Fine," I said, "I'll write the letter in some of the rest of my time."

When Lord Longford asked him nearly a decade later to write a novel about a third world war fought in 1985 would look like to the contemporary historians in 1987, Hackett saw another opportunity to lecture about NATO's declie.

Setting about his task with deadly seriousness and military efficiency, he quickly asembled a crack team of recently retired experts - "people off the hook, but not yet off the boil" - including another NATO general, a vice admiral, and air chief marshal, a British ambassador to NATO, and an economist. They produced piles of military analyses, feasibility studies, position papers, situation reports, contingency plans and battlefield maps not unlike those that must fill situation room files in NATO headquarters, the Pentagon and the Kremlin.

In just a few months, Hackett stitched these together into a fascinating if not particularly literary, scenario in which the Soviets take control of the Middle Eastern oil fields by proxy, instigate a black African invasion of South Africa, occupy post-Tito Yugoslavia and send an armada of tanks plunging through NATO defenses in Germany, the Netherlands and Italy.

In the end, however, the West manages to win the war with the help of uprisings against the Soviets in Eastern Europe, but only after London is blitzed again and an exchange of nuclear attacks obliterates both Birmingham, England (chosen instead of Chicago) and the Soviet city of Minsk.

Hackett seemed genuinely surprised and somewhat amused that htis became the stuff of a bestseller. Even beofre it was published this spring in the United States and quickly climbed bestseller lists. "The Third World War" sold 200,000 copies in Britian, Germany, The Netherlands, Denmark and Japan. French, Italian and Swiss translations also are coming out. Universal bought the screen rights and is preparing and costing out a script, but Hackett still thinks it would be much too expensive to film, dwarfing as it does a mere World War II battle like the Normandy landing.

Hackett does not allow himself to be modestly pleased by this sudden fame and financial success. Pointing out that a large share of the royalties will be collected in British taxes, he said the book "is making the chancollor of the Exchequer a lot of money." But he also thought that, because the book "caught the wave motion of the moment in public opinion," his "cautionary tale" might help bring about the significant streghtening of NATO ground forces in Europe that he believes is so badly needed and that is now being contemplated by the NATO nations themselves.

Hackett believes, along with many military men, that a European ground war today could easily lead wither to a Soviet victory or to a devastating, perhaps apocalyptic nuclear duel between the superpowers.

In fact, in Hackett's original scenario for the August 1985 war, based on his expert team's appraisal of likely relative strengths by then, the Russians rusched to the Rhine almost as quickly and easily as Hackett did in his 1967 war game victory. But when he showed that version to military and political friends in the United States and Europe, they feared its publication "would be harmful rather than helpful."

"We didn't want people to wet their beds," Hackett said. "We only wanted to get their attention. So I tore up 30,000 elegant words and started again."

This time, working on the assumption that both the U.S. and Britain increased their troop commitments in Germany, the U.S. replenished its depleted "immediate replacement" reserves by reinstituting the draft, and Britian greatly increased its defense spending and patched up its air deforcements from America, Hackett found a way for the West to squeak throug with just one city on each side destroyed by nuclear bombs.

"We have to make it possible to resort to arms without mutual destruction by nuclear weapons," Hackett explained. "This is my central thesis, and I believe it is a realistic one."

Lord Logford was more impressed by the sales success of "The Third World War" and begged Hackett to do a swquel - on the end of the world. But Hackett turned him down, explaining that he was not a expert on that subject.

"He took me to anice lunch at the Garrick Club to twist my ram," Hackett said, "because he was certain it would be another best seller. But I'm afraid I disappointed him when I said I was planning instead to write Byzantine Review articles after I go to Turkey and look at some of the places occupied there by the Knights of Saint John from their base on Thodes."

Hackett also busied himself with a series of six programs for BBC television tracing, Alistair Cooke-style, the long history of military medals and honors in Britian. "At one point, we had 70,000 pounds ($150,000) worth of gold in the studio," he recalled with childlike delight, "And I had the original Armada Medal from the British Museum in my pocket. I forgot and almost walked out with it until one of the big gorillas guarding the stuff stopped me."

As for the future, Hackett said, he looks forward to the time "when I can wake up in the morning and o nothing but read Moliere between breakfast and lunch. Diciplined selindulgence - that's the way I purpose to spend the rest of my days." CAPTION: Picture, Sir John Hackett