Clancy brought such realism and attention to detail to his novels that in 1985, a year after the Cold War thriller "The Hunt for Red October'' came out, a military official suspected the author of having access to classified material.
"Thrillers, like all art, are always a reflection of the culture,'' said fellow author Brad Meltzer. "No one captured that Cold War fear and that uniquely American perspective like Clancy. Jack Ryan wasn't just a character. He was us. He was every American in those days when we were a push-of-the-button away from nuclear war.''
The best-selling novelist, who died Tuesday in Baltimore at 66, insisted then, and after, that his information was strictly unclassified: books, interviews and papers that were easily obtained. Also, two submarine officers reviewed the final manuscript.
Government officials may have worried how Clancy knew that a Russian submarine spent only about 15 per cent of its time at sea or how many Seahawk missiles it carried. But his extreme attention to technical detail and accuracy earned him respect inside the intelligence community and beyond. It also helped make Clancy the most widely read and influential military novelist of his time, one who seemed to capture a shift in the country's mood away from the CIA misdeeds that were exposed in the 1970s to the heroic feats of Jack Ryan.
Fans couldn't turn the pages fast enough and a number of his thrillers, including "The Hunt for Red October,'' ''Patriot Games'' and "Clear and Present Danger,'' were made into blockbuster movies, with another Jack Ryan film set for release on Christmas Day.
"Fundamentally, I think of myself as a storyteller, not a writer,'' Clancy once said. "I think about the characters I've created, and then I sit down and start typing and see what they will do. There's a lot of subconscious thought that goes on.''
A tall, trim figure given to wearing sunglasses that made him look like a fighter pilot, Clancy had such a sure grasp of defense technology and spycraft that many readers were convinced he served in the military. But his experience was limited to ROTC classes in college. Near-sightedness kept him out of active duty.
In 1982, he began working on "The Hunt for Red October,'' drawing inspiration from a real-life 1975 mutiny aboard a Soviet missile frigate. He sold the manuscript to the first publisher he tried, the Naval Institute Press, which had never bought original fiction. In real life, the mutiny was put down, but in Clancy's book, a Soviet submarine skipper hands his vessel over to the U.S. and defects.
Someone thought enough of the novel to give it to President Ronald Reagan as a Christmas gift. The president quipped at a dinner that he was losing sleep because he couldn't put the book down _ a statement Clancy later said helped put him on the New York Times best-seller list.
"What happened to me was pure dumb luck. I'm not the new Hemingway,'' Clancy later said in an interview with the American Movie Channel.
"Of course, fortune does favor the brave. In battle, you forgive a man anything except an unwillingness to take risks. Sometimes you have to put it on the line. What I did was take time away from how I earned my living. My wife gave me hell. 'Why are you doing this' But she doesn't complain anymore.''
Clancy said his dream had been simply to publish a book, hopefully a good one, so that he would be in the Library of Congress catalog. His dreams were answered many times over, with worldwide sales of his books estimated to exceed 100 million copies.
Alec Baldwin, Ben Affleck and Harrison Ford have all played Jack Ryan on screen and "Jack Ryan: Shadow One'' is set to open on Christmas Day, starring Chris Pine as Ryan.
Clancy wasn't always happy about the movie versions of his books. He complained that Ford was too old to play Jack Ryan, and he regretted the lack of creative control, saying: "Giving your book to Hollywood is like turning your daughter over to a pimp.''
Clancy started off writing about the Russians, but also told stories of Latin American drug cartels, Irish-British tensions and Islamic terrorism. He wrote nonfiction works on the military and ventured into video games, with a number of best-selling titles.
His recent Jack Ryan novels were collaborations with Mark Greaney, including "Threat Vector'' and a release scheduled for December, "Command Authority.'' As of midday Wednesday, "Command Authority'' was No. 35 on Amazon's best-seller list.
Born in Baltimore on April 12, 1947, to a mailman and his wife, Clancy was fascinated by military history as a child. He entered Loyola College as a physics major but switched to English as a sophomore. He later said he wasn't smart enough for the rigors of science, though he clearly mastered it in his fiction.
After school, he worked in an insurance office that had military clients. By the early 1980s he had written a piece about the MX missile system that was published by the Naval Institute. Boredom with his job led him to try novels. He wrote daily and set a goal of five completed pages a day.