Blitzkrieg 3 [official site] is eventually going to have a substantial singleplayer campaign, but its more inventive and intriguing mode is its asynchronous multiplayer. That's what is available now via Steam Early Access. You spend resources to build bases which, even when you're offline, other players can then choose to attack. Come see the trailer below.
What a lot of explosions.
Blitzkrieg is a long-running strategy series but this is the first time that the multiplayer mode has been a central focus. The hope is that running that mode through Early Access will allow the team the time they need to polish and balance the mode, and currently a final release is planned for the Autumn which means they've four or five months to tinker away. Aside from that singleplayer campaign, the eventual final release should also bring "in-game Generals," which sound like your typical strategy hero unit, plus daily quests, new maps and leaderboards.
You can buy the game's standard edition for £23 and if you do so before May 12th, you also get Blitzkriegs 1 and 2.
For more, check out Pip's hands-on preview with the game. Don't look at its title however, as it will plant a song firmly in your head for the rest of the day.