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Hello Ms. Johnson,
Thank you for contacting History Hub regarding naturalization after military service. Here is a link to a public website, naturalizationrecords.com, that basically sums up that the soldier was not granted citizenship but that the application requirements were greatly reduced with honorable discharge from service. Laws applicable to immigrant military veterans were passed and amended between 1862 - 1952.
Original applications and certificates are available to order from USCIS.gov or from the regional National Archives that hold courts records where the soldier processed his application. You may also find naturalization applications or supporting documents on familysearch.org or ancestry.com.
I hope this information is helpful. Best of luck with your research!