Retired Navy Adm. Robert Lee Dennison, 78, a veteran of World War II and naval aide to former president Harry S. Truman, died of a pulmonary embolism Friday at Bethesda Naval Hospital.

During his last three years before retiring in 1963, Adm. Dennison was commander-in-chief of the U.S. Atlantic Fleet and Supreme Allied Commander, Atlantic (SACLANT) for NATO.

These years included the abortive invastion of Cuba by anti-Castro forces at the Bay of Pigs in 1961 and the U.S. naval blockade of Cuba a year later.

Adm. Dennison, as part of an oral history project, explained his role in the Bay of Pigs operation. He said that he found out about the planned invasion nearly by accident; said that it was based on political assumptions concerning Cuba that were out of date, and added that he was forbidden to attack ground targets in support of the invasion.

"Here were these poor expeditionary guys getting cut up, and I couldn't blast the Cuban forces that were attacking them. All I could do was to give air cover to protect them against air attack only," he said.

"I finally got a directive to have destroyers take people off the beach and give them cover . . . We picked up quite a few people."

During the "Quarantine" of Cuba in November 1962, an operation whose purpose was to prevent the Soviet Union from sending offensive missiles to Cuba, Adm. Dennison served as commander of the U.S. Unified Command that enforced it.

The Joint Chiefs of Staff gave him command of all Army, Navy, Air Force and Marine Corps forces assigned to the action -- a force of more than 300,000 men.

When he retired in May 1963, President John F. Kennedy awarded Adm. Dennison the Detinguished Service Medal.

Adm. Dennison was a native of Warren, Pa., and a 1923 graduate of the Naval Academy at Annapolis. He earned a postgraduate degree in diesel engineering from the Naval Academy, a master's degree from Pennsyvania State University and a doctoral degree in engineering from Johns Hopkins University in 1935.

Before World War II, Adm. Dennison commanded submarines and destroyers and was a member of the staff of Adm. Thomas Hart, commander of the old Asiatic Fleet, based in thePhilippines, when Pearl Harbor was attacked.

Moving to Australia with the remnants of the Asiatic Fleet, Adm. Dennison became chief of staff to the commander of a Pacific fleet amphibious force.He participated in the planning, seizure and occupation of Attu and Kiska in the Aleutians in 1942. He was awarded the Legion of Merit.

Following the war he was named assistant chief of naval operations for politico-military affairs a post he held from 1946 to 1947. He later commanded the battleship Missouri for a short time before becoming President Truman's naval aide in 1948.

During his five-year stint in the White House, he was promoted from captain to rear admiral and according to newspaper accounts of the time, acted as an adviser to Truman and was relieved of the post's usual assignment boards and ceremonial duties.

Adm. Dennison later was commander of the Pacific Fleet and deputy chief of naval operations for plans and policy before becoming commander of Naval Forces for the eastern Atlantic and Mediterranean in 1959.

He joined the Copley Press in June 1963 and was chairman of the board of the Copley Computer Services for four years before retiring a second time in 1973.

He had lived in Washington since 1963.

In addition to his other medals, Adm. Dennison held a second Legion of Merit.

He was a director of the Naval Historical Foundation, and belonged to the Army-Navy, the Chevy Chase and Metropolitan clubs.

Survivors include his wife of 42 years, the former Mildred Fenton Mooney Neely of Washington; a daughter, Lee Dennison of San Diego; a son, Robert Lee Jr. of Charleston, S.C., and one grandchild.