Riordan joined the Army from
Kansas City, Missouri in 1940, and by February 3, 1944, was serving as a
second lieutenant in the
133rd Infantry Regiment,
34th Infantry Division. On that day, near
Cassino,
Italy, Riordan single-handedly silenced a
German machine gun emplacement. Five days later, on February 8, he was again in the lead during an attack on a German strongpoint. Cut off from his unit, he attempted to take the objective on his own, but was killed in the process. He was posthumously awarded the Medal of Honor on September 11, 1944. Riordan, aged 23 at his death, was buried in Mount Olivet Cemetery,
Raytown, Missouri. ==Medal of Honor citation==