John S. D. Eisenhower, the son of President Dwight D. Eisenhower who forged a reputation in his own right as a military historian, died on Saturday at his home in Trappe, Md., on the Eastern Shore. He was 91.

The death of Mr. Eisenhower, who had been the oldest surviving child of an American president, was announced by his family.

Mr. Eisenhower was an Army officer in World War II and the Korean War and a national security adviser during his father’s presidency, but at that point he was still viewed primarily as the son of Ike, the American hero.

When he graduated from West Point on June 6, 1944, photographers gathered for an image of Mr. Eisenhower, the new second lieutenant, with his mother. But he was seen as a footnote to a historic day: Gen. Dwight D. Eisenhower, the supreme Allied commander, had just announced the D-Day landings on the beaches of Normandy.