From the Prologue:

A total of two hundred and eight American soldiers served with the 400th USASA SOD (Abn) between 1962 and 1975. The US Army Security Agency assigned the men of this unit both Top Secret classified and covert missions. Their operations were conducted under surreptitious conditions for security reasons. All of the 400th ASA SOD members were three time volunteers: first of all, they volunteered for their U.S. Army assignments; secondly, each had to volunteer and successfully complete the US Army Airborne Training School; and thirdly, each had to volunteer to serve with the new ASA Special Operations Detachments.