Although I've tried to make all terms understandable through context, the following are terms or expressions modern readers may find unfamiliar.
Archie: Short for Archibald, common slang for anti-aircraft artillery
Boche: Derogatory term for German
Chasseur: Infantry or cavalry soldier trained for rapid movement
Deuxième Bureau: French Army Intelligence
Hun: Derogatory term for German
Jasta: Abbreviation of jagdstaffel, term for a German squadron of fighter planes
Kepi: Round military cap with a visor
Marraine de guerre: A godmother for a French soldier
Poilu: Literally “hairy one,” term for a French infantryman
Rationer: Soldier responsible for rations