Dear Ms. Mills,
Thank you for posting your request on History Hub!
The National Archives and Records Administration (NARA) is the depository of the permanently valuable non-current records of the United States Government. We suggest that you contact the Polish Digital Archives, the Polish State Archives as well as the German Federal Archives for information about Asher and Rosalie Mendelsohn. Also, please review the FamilySearch Research wiki for Poland Genealogy and Germany Genealogy.
Also, you may wish to search Ancestry, FamilySearch, and newspaper archives for mention of Asher and Rosalie Mendelsohn. There may be a fee for using these services. Instead, you may wish to contact your local library. Many library systems subscribe to these sites, making them free for patrons.
We hope this is helpful. Best of luck with your research!