Henry A. Prunier taught Vo Nguyen Giap, the Vietnamese general who withstood the armies of France and the United States, how to throw a grenade.

The lesson came in July 1945, after Mr. Prunier and six other Americans had parachuted into a village 75 miles northwest of Hanoi on a clandestine mission to teach an elite force of 200 Viet Minh guerrillas how to use modern American weapons at their jungle camp.

The Americans, members of the Office of Strategic Services, the United States’ intelligence agency in World War II, wanted the guerrillas’ help in fighting the Japanese, who were occupying Indochina. The Viet Minh welcomed the American arms in their struggle for Vietnamese independence....