Dear Mr. Anderson,
Thank you for posting your request on History Hub!
We located the following resources concerning the history of the 91st Division during World War I. None mention the division as a whole being awarded a Belgian Croix de Guerre. Also, the earliest US Army unit award, the Presidential Unit Citation, was not introduced until 1941 and was not applied retroactively to World War 1.
We searched the National Archives Catalog and located a series titled Records of Divisions, 1917 - 1920 in the Records of the American Expeditionary Forces (World War I) (Record Group 120) that includes records of the 91st Division (in P 1241-91) and may include information about individual awardees. For more information about these files, please contact the National Archives at College Park - Textual Reference (RDT2) via email at email@example.com.
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We hope this is helpful. Best of luck with your research!