The family of President Dwight D. Eisenhower says his son, 91-year-old John S.D. Eisenhower, has died.

The retired brigadier general and military historian had resided on Maryland’s Eastern Shore in the community of Trappe, Md. No cause was given for his death Saturday.

John Sheldon Doud Eisenhower was born in Denver in 1922 to Dwight D. Eisenhower and his wife, Mamie. He was an assistant staff secretary in the White House during the last few years of his father’s administration, and served as U.S. ambassador to Belgium during the Nixon administration.

Eisenhower was the author of numerous books about military history.

He is survived by his second wife, Joanne, son David and daughters Anne, Susan and Mary.