While it may be that Social Security numbers were not required at one time, I believe that military service numbers now are Social Security numbers. I know for a certainty that my old military number before I retired is no longer used by the Army. It uses Social Security Numbers as military ID numbers. It seems to follow that if a person wants to enlist they must have a Social Security number that then will be used to be their military ID number. Perhaps there are variations on this , but I believe that SS. numbers are now required for some people.