I'm rather unclear on the finer points of WWI military structure. I have some information on the service of my half-great-uncle Harry Brosius, but I can't figure out how to categorize it. The information I have:
On 26 Mar 1918, Harry enlisted from Tucumcari, New Mexico, in the U.S. Army. He began as a private in Company F of the 30th Engineers, and departed from Hoboken to Europe aboard the President Grant on 30 June 1918.  According to the Brosius genealogy book by Lewis W. Brosius, Harry himself related that he "was sergeant in Gas Guard of Chemical Warfare Service in 89th Division and was in St. Mihiel and Meuse-Argonne drives and with the army of occupation on the Rhine." At the end of the war, he was a corporal in the 6th Casual Company Chemical Warfare Service, and departed from Le Havre aboard the S.S. La Lorraine on 23 Mar 1919. He was discharged on 18 Apr 1919.
I can't find the Chemical Warfare Service in the category structure, nor the 89th Division, and when it comes to the "30th Engineers," I don't know if that's a regiment, battalion, brigade, or what. I will definitely appreciate some advice! Thank you.