If your subject was in the US armed services, see:

An Overview of Records at the National Archives Relating to Military Service

Access is different if you're the veteran, or a family member of the veteran, as opposed to searching for records of someone unrelated to you. See:

If you are not family:

Access to Official Military Personnel Files (OMPF)for the General Public

If you are the veteran, or next-of-kin

Access to Official Military Personnel Files (OMPF)Veterans and Next of Kin

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