Dear Mr. Leas,
Thank you for posting your request on History Hub!
We searched the National Archives Catalog and located 5 series, 8 file units, and 38 items that pertain to the Chosin Reservoir. Please contact the reference unit indicated at the bottom of the descriptions.
For more information about your grandfather’s military service, please request a copy of his Official Military Personnel File (OMPF). OMPFs and individual medical reports for enlisted men of the U.S. Army who were separated from the service after October 1912 and prior to 1957 are in the custody of NARA's National Personnel Records Center in St. Louis. In many cases where personnel records were destroyed in the 1973 fire, proof of service can be provided from other records such as morning reports, payrolls, and military orders, and a certificate of military service will be issued. Please complete a GSA Standard Form 180 and mail it to NARA's National Personnel Records Center, (Military Personnel Records), 1 Archives Drive, St. Louis, MO 63138-1002.
We hope this is helpful. Best of luck on your research!