On Monday, June 25, 1973, about 7 p.m., a Polish army lieutenant colonel slipped into an unremarkable apartment in Hamburg. Soon he was face-to-face with a stranger, an American in wire-rimmed glasses who was introduced to him as “Daniel.” The American sought to put him at ease, speaking to him in Polish: “I’m from the CIA, and I’m delighted to be here to work with you.”

They talked, and the Polish agent returned the next day. The apartment was a CIA safe house. The American had a long list of questions for the Polish agent that went to the heart of the Cold War military confrontation in Europe. Smoking a cigarette and sipping fruit juice, the Polish agent provided remarkable details about the military thinking of the Soviet Union and the Warsaw Pact, according to Benjamin Weiser, author of a 2004 book on the case, “A Secret Life.”

The agent feared that his beloved Poland might be drawn into a nuclear war in Europe, incinerated as a buffer zone for Soviet overlords. Motivated by disgust at the Soviet domination of Poland and its military, the Polish agent had begun spying for the Americans.

The meetings were the first contact between Ryszard Kuklinski, the Polish agent, and David W. Forden, who became his CIA case officer, a confidant who guided Kuklinski’s espionage during one of the most productive and significant intelligence operations of the Cold War. Mr. Forden, 88, died Feb. 12 at an assisted-living and memory-care center in Alexandria, Va. The cause was Alzheimer’s disease, said his daughter, Sara Forden.

Mr. Forden played a key role in the Kuklinski operation, especially in its first years. The work of a case officer in managing an agent, especially in “denied areas” of the Cold War such as Poland, meant operating in a tense environment of near-constant surveillance. The case officer had to develop trust with the agent, become a friend and confessor, and serve as the agent’s adviser and protector, while providing equipment, training and feedback, and reporting frequently back to headquarters.

When they first met, Mr. Forden and Kuklinski clicked immediately. “Time was short,” Mr. Forden recalled last year in a statement he prepared for a Polish ceremony to honor Kuklinski. “We took off our jackets, rolled up our sleeves and got down to work.”

“We didn’t recruit him, he recruited us,” Mr. Forden added. “Long before I met him, Kuklinski had already decided that Poland deserved to be a free state and that there was only one country that could help achieve that goal — the United States.”

After the first meeting in 1973, Mr. Forden cabled back to headquarters: “He is a valiant, able and dedicated man, who, in my view, does not consider himself a ‘traitor’ or the participant in some kind of ‘dirty game.’ He is ‘stabbing back’ at those who have made a shambles of his country, and he expresses deeply serious satisfaction in the fact that he can do this by cooperating with us.”

A key question about any spy is motive. “He wasn’t interested in personal glory or status,” Mr. Forden recalled last year in his statement, “but in a new horizon for Poland.”

Over the years, Kuklinski provided the United States with copies of secret Soviet war plans in Europe, Warsaw Pact military deployments, a journal of high-level Soviet military doctrine and thinking, and early warning about a possible Soviet military intervention in response to the rise of the Solidarity labor movement in Poland.

A CIA report quoted by Weiser said that as of July 1981, Kuklinski had provided 40,265 pages of highly classified Soviet documents, passed in 63 separate exchanges with the CIA. He was described as “the best-placed source now available to the U.S. government in the Soviet Bloc in terms of collection of priority information.”

Kuklinski was perfectly positioned to spy on the Warsaw Pact and Soviet Union because he was assigned to the Polish general staff, where he could read and photograph Soviet war plans. “Thanks to Kuklinski, we had the information and insight to know the difference between military exercises and warfaring escalation,” Mr. Forden recalled in his statement. “Without the perspective he offered, history may well have taken a completely different course.”

Mr. Forden was among a generation of CIA officers who joined the agency after World War II, as the Cold War gathered steam. They were eager to carry out aggressive and innovative techniques to communicate with agents behind the Iron Curtain. Often these techniques involved using illusion and concealment, moving through a “gap” in the surveillance of a hostile intelligence service.

Mr. Forden’s contribution included a technique known as the car pass, in which a packet of film or information could be transferred between moving vehicles after they turned a corner without being detected by hostile surveillance teams.

In 1981, Kuklinski was exfiltrated to the United States, and he and Mr. Forden remained close as he settled into American life. Kuklinski died in 2004. Mr. Forden received the CIA’s Distinguished Intelligence Medal in October 1988 and retired that December. The Kuklinski operation was dramatized in a 2014 Polish film, “Jack Strong,” a code name that Kuklinski used in signing messages to the CIA.

Kuklinski loved sailing and served as captain of a Polish general staff yacht that he piloted to Germany, where he first volunteered to spy for the United States in a letter sent to the American embassy in 1972. When Kuklinski eventually came to the United States, he brought Mr. Forden a piece of art that had once hung in his home, an 1899 etching of a sailing ship that lay on its side, half-submerged off a sandy shore, and titled “After the Storm.” Mr. Forden hung it proudly and showed it to visitors in his home in Arlington.

David Warner Forden was born in Buffalo on Sept. 11, 1930, and grew up in East Aurora, N.Y. His father was an office clerk, and his mother was a homemaker.

He entered Wesleyan University on scholarship and graduated in 1952 with a bachelor’s degree in government. The next year, he received a master’s degree from Syracuse University’s Maxwell School of Citizenship and Public Affairs, where his roommate was future “Columbo” actor Peter Falk.

He was offered a job at the CIA in 1954, but first completed Officer Candidate School at Fort Benning, Ga., after which he served a six-month tour as a paratrooper with the Army’s 82nd Airborne Division, then returned to the CIA for training in intelligence operations.

Over the course of his CIA career, he was stationed in Germany, Argentina, Poland, Mexico, Austria and Greece, and he held key assignments at headquarters, including head of the CIA’s Soviet/East Europe division.

His first marriage, to Sally Carson, ended in divorce. In 1984, he married Aurelia Bachmeyer, who died in 2003. Survivors include three children from his first marriage, Sara Forden of Arlington, Daniel W. Forden of Evanston, Ill., and Katherine Forden of Berlin; a sister; and four grandchildren.

In 2005, Mr. Forden told a Wesleyan alumni publication that he did not initially intend on having a CIA career. He came to Washington looking for a government job through the university’s strong career network but found most agencies closed off amid a federal hiring freeze.

One of the exceptions was the CIA amid the Red Scare. Ironically, he said, a recruiter had once come to campus and noted in a file that Mr. Forden “didn’t have the stamina for a career in clandestine services.”