Is the game multiplayer-only?

[Morak]

Does the game have bots/singleplayer? Or is it only online multiplayer?

Also is it possible to host lan games?

[Beef Squisher]

The main game is multiplayer. There is a mode with bots, it's a hoard mode sort of thing. Overall if you're looking for a single player game this is not it, there is no campaign or anything like that.

I don't think you can host LAN games (not 100% on this point, around 69% sure), but you can host private games

[LV|Ironstorm767]

Can't host LAN games, but you can play Squad Defense (1-4 players against waves of bots) in the game's Offline Mode(*).

However, from the latest News announcement, the Devs are considering adding bots to fill Frontlines (the main trench warfare mode) matches when a regional server's player population is low.

It would allow players to create private matches with bots filling the match, and I could see the same applying for the game's Offline Mode. (Pure speculation on the latter)

(*)Note: Don't believe that using Steam's offline mode will start the game's Offline Mode and would need to be disconnected from the Internet. (It needs more votes)[support.ww1gameseries.com]

IMO however, if you want more single player support, you could check out the second game by Verdun's developers: Tannenberg. (It's in Early Access, but has better bot + offline support.)