Stevens enlisted in the Navy on Dec. 6, 1941, hours before the Japanese attacked Pearl Harbor. He later won a bronze star for his service as a cryptographer, after he helped break the code that informed American officials that Adm. Isoroku Yamamoto, the commander of the Japanese Navy and architect of the Pearl Harbor attack, was about to travel to the front.

I haven't read this entire interview, but this data point might be of interest to several folks here. Its not clear from this whether he had a scientific or operational role.

Justice John Paul Stevens broke a Japanese Cipher