Thank you for posting your request on History Hub!
The American Battle Monuments Commission is a good start for explanatory information on the Battle of Belleau Wood https://www.abmc.gov/Belleau-Wood
Also, the USMC University has a helpful explaining who the commanders were and number of causalities https://www.usmcu.edu/Portals/218/HD/Brief%20Histories/Belleau%20Wood.pdf?ver=2019-05-23-083625-560
The USMC website also includes their actions in the battle https://www.6thmarines.marines.mil/Units/1st-Battalion/History/
If you enter the name of the battle in out catalog, you will also come across images such as the one at this link https://catalog.archives.gov/id/31483044 which shows funerals near the site.
In 1973, Corporal Elton Mackin of the 67th Company 1st Battalion 5th Marine Regiment gave an interview, which is transcribed here https://www.rbhayes.org/research/corporal-elton-mackin-wwi-5th-marine-regiment-interview-1973/
Also, Colonel Albertus W Catlin https://www.history.navy.mil/our-collections/photography/us-people/c/catlin-albertus-w.html wrote a book about his experiences titled "With the Help of God and a Few Marines": The Battles of Chateau Thierry and Belleau Wood
The Marine Corps Times has a recent article during the centennial of the battle that mentions letters written about the battle https://www.marinecorpstimes.com/news/your-marine-corps/2020/11/04/these-are-the-marines-who-died-at-belleau-wood/
And here is a link to the publication Army History from 2003 https://history.army.mil/armyhistory/AH58newOCR.pdf
We hope this helps in your research!
Dear Ms. Sherwood,
We located some additional information that may be helpful in your research.
We searched the National Archives Catalog and located the series Records of Divisions, ca. 1918 - 1942 in the Records of the American Expeditionary Forces (World War I) (AEF) (Record Group 120) that contains records related to a variety of military units that served during World War I, including (among others) infantry regiments, machine gun battalions, and marines, so there may be records of the particular American units that fought at Belleau Wood within the series. These records have not been digitized. Please contact the National Archives at College Park - Textual Reference (RDT2) at email@example.com for access to and information about these and similar records. Please include as much information (e.g. specific units, date ranges, etc.) as possible in your inquiry.
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 RDT2. We apologize for this inconvenience and appreciate your understanding and patience.
Again, we hope this information is helpful, and best of luck with your research!