There are very few games set in this time period (at least vs. WWII titles).

Singleplayer :- Valiant Hearts is an emotion-driven and beautifully presented game that deals with World War 1. It is not a war game but a game about war. A story inspired by letters written in the First World War. Valiant Hearts: The Great War follows 4 characters and a dog as they attempt to survive the horrific conflict by relying on each other as their journeys intersect. Beautiful hand drawn comic book style animation powered by the UbiArt Framework. It combines puzzle-adventure gameplay with great storytelling and imparts knowledge of the historical background. WW1 is one of the most bloodiest wars in the history of mankind. Thousands upon millions gave their lives for their country and for the greater good despite the conflict of bloodshed. Stories are told to this day over the war and it's horror, and this is one of them. I leave you with the beautiful end theme.....

Multiplayer :- Verdun is a tactical squad-based WW1 trench warfare inspired by the infamous battle of Verdun in 1916. The Battle of Verdun began in February 1916, and lasted ten months. It was the lengthiest battle during World War I, claiming around 700,000 dead, wounded, or missing soldiers. Verdun is a great FPS made of mud, trenches, gas warfare and death. Many other games have a similar capture and hold this area gameplay mechanic but Verdun takes a different approach. Good selection of weapons and squad roles. Amazing progression system, makes you want to work for it. The game doesn't have the best graphics, the best character models, or what have you—and yet few games have ever captured the feel of trench-based warfare and so consistently stressed me out like Verdun. For years WW2 shooters were all the rage in video games. It is a rare shooter set in a time period more frequently visited by strategy games. Thanks to the team that is making this it keeps getting better and better. Putting AAA games to shame with free DLCs during development.

With how important the Great War was, it's satisfying to see smaller developers (or small teams within larger developers, in the case of Valiant Hearts) getting the chance to explore this tragedy through more niche games.

Mods :- The Great War 1918 (COH mod), Over The Top Arma WW1 mod. Battlefield 1918 mod was great - for a Battlefield 1942 mod.