Part of the process of becoming a US citizen is renouncing one's allegiance to the country of which they were a former citizen. You would need to know if your grandfather became a US citizen.
Dear Mr. Harmon,
Thank you for posting your request on History Hub!
Since the U.S. Census notes citizenship status, we suggest you check the 1930 and 1940 Census for the status of your grandfather. We searched the National Archives Catalog and located the Population Schedules for the 1930 Census, and the Population Schedules for the 1940 Census in the Records of the Bureau of the Census (Record Group 29) that may contain records of your grandfather’s citizenship. The 1940 Census schedules are digitized and available using the Catalog. See NARA’s 1940 Census Records web page for more information. For access to the non-digitized schedules, please contact the National Archives at Washington, DC - Textual Reference (RDT1) via email at firstname.lastname@example.org.
Due to the COVID-19 pandemic and pursuant to guidance received from the Office of Management and Budget (OMB), NARA has adjusted its normal operations to balance the need of completing its mission-critical work while also adhering to the recommended social distancing for the safety of NARA staff. As a result of this re-prioritization of activities, you may experience a delay in receiving an initial acknowledgement as well as a substantive response to your reference request from RDT1. We apologize for this inconvenience and appreciate your understanding and patience.
For information about the U.S. Census, see the Census Bureau technical documentation and questionnaires. Please review the FamilySearch wiki for Naturalization Terms and Acronyms and NARA’s Naturalization Records website for an overview of the naturalization process.
You may wish to search Ancestry.com or FamilySearch.org for the U.S. Census. There may be a fee for using Ancestry. Instead, check for access at your local library as many library systems subscribe to these sites, making them free for their patrons.
If your grandfather was listed as a citizen in a U.S. Census, the naturalization would have taken place in a Federal court. Naturalization indexes, declarations of intention (with any accompanying certificates of arrival), and petitions for naturalization will usually be in the custody of the National Archives facility serving the state in which the Federal court is located.
Unfortunately, no central index exists. To ensure a successful request, please include the name of petitioner (including known variants); date of birth; approximate date of entry to the US; approximate date of naturalization; where the individual was residing at the time of naturalization (city/county/state); and country of origin.
In most cases, the National Archives will not have a copy of the certificate of citizenship. Two copies of the certificate were created – one given to the petitioner as proof of citizenship, and one forwarded to the Immigration and Naturalization Service (INS).
Certificates of citizenship were issued by the Federal courts until October 1991 when the Immigration and Naturalization Service (INS) took over responsibility for naturalization proceedings. All INS records are now overseen by the US Citizenship and Immigration Services (USCIS). USCIS maintains duplicate copies of court records (including the certificate of citizenship) created from September 27, 1906 to March 31, 1956 within Certificate Files (C-Files). C-Files can be requested through the USCIS Genealogy Program.
If you find no citizenship status listed for your grandfather in the U.S. Censuses, we suggest that you request a certification of non-existence of a record of naturalization from USCIS (Genealogy Frequently Asked Questions - see About Further Research section), as well as submit an Index Search request to the USCIS Genealogy Program. USCIS should be able to provide the certification on non-existence, and although your grandfather won’t have a naturalization record, it is likely that INS did create an A-File that could be used to verify his citizenship status.
We hope this is helpful. Best of luck with your family research!