Military unit associations
I was recently asked if there were records containing rosters of Army Airborne trainees at Fort Benning during World War II. There is no simple answer. However, there is an association dedicated to paratroopers. They may not have the records being sought but their members constitute a brain trust that might be able to point the person asking the question in the right direction.
I served in Vietnam aboard a Navy guided missile destroyer escort and as I searched around, found an association made up of former crew members of my ship! Many military units from all branches of the service have active associations that a researcher can contact to get some basic information about where different units served and even who some of the key personnel were at specific times. Units often produced unit histories containing tons of good information.
If a person knows what type of soldier (Marine, airman, or sailor) the ancestor was, there is likely to be an association out there that can help identify the location of specific units at specific times and thereby provide a finding aid for more information to aid in identifying the specific unit in which an ancestor might have served.
For example, google "destroyer associations" or "paratrooper associations," or some other descriptive term. Be creative and you might be surprized what surfaces. Happy hunting!
It can't hurt!