Tom Clancy, a former Maryland insurance agent who became the most popular military novelist of his era, entertaining millions of readers and stunning Pentagon officials with his uncannily authentic accounts of submarine warfare and covert operations, died Oct. 1 in Baltimore. He was 66.

His publisher, G.P. Putnam’s Sons, confirmed his death but did not disclose the cause.

Mr. Clancy bolted from obscurity to international attention in the 1980s as the defining military writer of the Cold War. Through his pop fiction, he later became a key chronicler of the counterterrorism age.

His brick-size books sold tens of millions of copies and spawned the tactical-shooter genre of video games that in recent years has come to dominate the billion-dollar­ industry. Several volumes, including his debut novel “The Hunt for Red October” (1984), “Patriot Games” (1987) and “Clear and Present Danger” (1989), inspired movies featuring Hollywood stars such as Sean Connery, Alec Baldwin and Harrison Ford.

Critics dismissed Mr. Clancy’s works as fare to be bought at the supermarket and consumed in airport terminals and on beaches. They faulted him for what they regarded as his failure to present human beings with as much nuance as he depicted submarines and tanks.

Beyond the pulpy covers of his paperbacks lay staggeringly accurate descriptions of military weaponry and strategy. John F. Lehman Jr., who served as secretary of the Navy during the Reagan administration, read “The Hunt for Red October” shortly after its publication and later spoke to the author.

“The first thing he asked me about the book,” Mr. Clancy told the New York Times, “was ‘Who the hell cleared it?’ ”

Mr. Clancy joined the ROTC during college but was precluded from service in the Vietnam War because of his pronounced nearsightedness, an impairment betrayed by his recognizable dark-tinted Coke-bottle glasses. He settled on a career in insurance sales but never abandoned his interest in the military — or his childhood hope of becoming a writer.

By the early 1980s, he had finished in his spare time the manuscript for “The Hunt for Red October,” an account of the defection of a Soviet submarine captain and the ensuing confrontation with the United States in the North Atlantic. The Naval Institute Press in Annapolis took a risk on the manuscript — and on Mr. Clancy, its first author of original fiction — and bought it for $5,000.

At first, the book was circulated among government leaders in Washington, who the publishing house hoped would enjoy its combination of accuracy and intrigue. The volume made its way to President Ronald Reagan, who was quoted as saying that he could not put the book down. That endorsement helped propel “Red October” to the top of the sales charts.

For nearly three decades that followed, Mr. Clancy fired off one thriller after another. His second volume, “Red Storm Rising” (1986), a ­hyper-technical account of an oil crisis that precipitates a third world war in Europe, became required reading at the Naval War College.

Reviewing Mr. Clancy’s book in The Washington Post, military historian John Keegan wondered “whether the Pentagon wouldn’t be spending some of its millions better at employing him inside Cheyenne Mountain” — the military bunker in Colorado — “than letting his talents go to waste in mere best-selling authorship.”

Mr. Clancy said that his information came from military journals and technical manuals — as well as from less traditional sources, such as “Harpoon,” the game based on modern naval combat. His research allowed him to write acronym-laden paragraphs that were either impenetrable or entrancing, depending on the reader’s interest in military arcana.

Some detractors said that the technology he described was more effective in Mr. Clancy’s fictional universe than in the real one. But at times, the author felt vindicated.

“During Desert Storm, I had a half-million reporters call me to say, ‘Gee, you were right all along, this stuff really works,’ ” he told The Post. “What do you think, I lied? I’ve used all this stuff. Of course it works.”

Readers opening his books and expecting a layered discussion about American military action may have been disappointed. To fans within and outside the military, Mr. Clancy captured not only the technical nature of weaponry but also the patriotism that inspired its use.

He filled his volumes with the exploits of characters such as John Kelly, a.k.a. Mr. Clark, a decorated former Navy SEAL doing work for the CIA.

His best-known hero, Jack Ryan, is a Wall Street stockbroker turned CIA operative. In “Patriot Games,” Ryan battles a fictional left-wing faction of the Irish Republican Army in a series of showdowns ending with a shootout at Ryan’s home. If the terrorists were lacking in texture, Mr. Clancy intended them to be that way.

“These guys are out committing murder, and you should treat them like murderers,” Mr. Clancy once told an interviewer. “Don’t say they’re soldiers; don’t say they’re politically motivated activists. Because they’re not. They’re street hoods. They’re scum.”

In “Debt of Honor” (1994), Ryan is serving as national security adviser during a series of emergencies including the crashing of a Japanese airliner into the U.S. Capitol building, a plot point that seemed eerily prescient after the terrorist attacks of Sept. 11, 2001. Earlier in Mr. Clancy’s career, Keegan, the military historian, had compared him to Jules Verne and H.G. Wells and other creators of “military futurology in Western literature.”

Mr. Clancy compared himself, in some respects, to William Shakespeare.

“Shakespeare wrote for the masses, and he wrote to make money,” Mr. Clancy told The Post. “He didn’t know he was turning himself into the greatest man in the English language. All he did was, he was trying to tell good stories that ordinary people could understand and give himself a decent living out of it. Well, I’m in the same tradition. I don’t put myself alongside the Bard for a lot of reasons — like I’m not that good, for one — but it’s an honorable tradition.”

Thomas Leo Clancy Jr., a Baltimore native like Jack Ryan, was born April 12, 1947. His father was a letter carrier and his mother was a department-store employee. Mr. Clancy grew up playing with ships, tanks and airplanes, activities that presaged the tales of war at sea, on land and in the air that he would tell as an adult.

He attended local Catholic schools before he received a bachelor’s degree in English in 1969 from what is now Loyola University Maryland in Baltimore. He had early ambitions of becoming a writer but encountered a setback when a science-fiction magazine rejected his first story.

He described himself as comfortable but bored in his job at the insurance company where he worked for his wife’s grandfather. (He and his wife, the former Wanda Thomas, later bought the firm.). In his free time, Mr. Clancy read military journals published by the Naval Institute, among others.

“I didn’t have anybody to blame but myself,” he told an interviewer. “I’d made my own trap, I had kids to support, mortgage payments and a business to pay off.”

Some of his insurance clients were Navy officers, and Mr. Clancy relied on them to confirm the accuracy of some of his writing. When he was working on “Red October,” he realized that he had a problem. He said he did not know who would board a submarine — that is, “what kind of person goes to sea in a ship that’s supposed to sink.”

A naval officer enlightened him.

“I found out that they’re pretty much the same as fighter pilots,” Mr. Clancy told an interviewer, “insofar as they have the same indecent sense of personal invincibility, the same kind of confidence, the ‘scarf and goggles’ effect. I never realized that. When I found it out I said, ‘Well, gee, I know test pilots. I’ve met an astronaut. If they’re just like that, I can write the story.’ ”

His novels included “The Cardinal of the Kremlin” (1988), “The Sum of All Fears” (1991), “Executive Orders” (1996), “Rainbow Six” (1998) and, more recently, “Against All Enemies” (with Peter Telep in 2011) and “Threat Vector” (with Mark Greaney in 2012). His book “Command Authority” is slated for publication in December.

In addition to his works of fiction, he wrote several works of military nonfiction.

Mr. Clancy’s first marriage ended in divorce. Survivors include his wife of 14 years, Alexandra Llewellyn, and five children. His home in Calvert County was reported to have been distinctly similar to the fictional estate belonging to Jack Ryan. Mr. Clancy was a part owner of the Baltimore Orioles.

When critics charged that his American heroes were too swashbuckling or that their adversaries were made of cardboard, Mr. Clancy stood by his characters.

“The guys in uniform are like cops and firemen. They’re basic, solid people, and they’re in the business of risking their lives for people they don’t know,” he told Publishers Weekly. “People who fight wars are the smartest people I know. . . . They’re also the happiest people I know.”

T. Rees Shapiro and Debbi Wilgoren contributed to this report.