After World War II in 1945, the American Occupation forces arrested Nagano. He was charged with Class A war criminal charges before the International Military Tribunal for the Far East in Tokyo. When US naval officers interrogated him, he was described as "thoroughly cooperative," "keenly alert," "intelligent," and "anxious to develop American friendship." The last Fleet Admiral of the Imperial Japanese Navy, he died of a heart attack due to complications arising from pneumonia in Sugamo Prison in Tokyo before the conclusion of his trial.