Cassidy entered the Army in January 1966 after graduating from Olney High School. The sergeant began his first tour in Vietnam in August 1966and his second in February 1968, serving as a rifleman and team leader with Company A of the 1st Battalion, 505th Infantry, 1st Brigade, 101st Airborne Division. Cassidy, 21, died near Hue, Thua Thien Province on March 20, 1968. He was posthumously awarded the Silver Star. Survivors included his parents, a brother, and 4 sisters.