Honestly, try Battlefield 1942 on single player. It's not half bad - the AI is about as smart as you would expect, but it's still fun to fight them. Also, the multiplayer is still going if you wanna try that!
Also, not WW2, but I HIGHLY recommend giving Vietcong a try (both the base game and Fist Alpha). It's one of the most engrossing games I've ever played. If you can get past the derpy (by today's standards) character models, this game has a seriously compelling campaign and there are still occasional multiplayer matches. If there was ever a game to make the Vietcong and NVA scary as fuck, it's that one.