AI War is a one-of-a-kind strategy game that plays like an RTS but feels like a 4X. With tower defense and grand strategy bits, too.
AI War: Fleet Command is a game that you can either play solo, or in 2-8 player co-op. You always play against a pair of AIs, and you can configure an enormous amount of things about the experience. The AI is very excellent, and also an entirely unique concept. The longer you play, the more the AI will impress you, which is kind of backwards from most strategy games, right? We named this "AI War" for a reason.
The only way to play is in procedurally-generated "campaigns." There are quite literally billions of possibilities, and every campaign has a really different feel to it. They also last a good while: most between 7 to 13 hours, about the length of time it takes to play through an FPS campaign once. This means your decisions -- victories and blunders alike -- have really long-term, interesting consequences. Some players talk about feeling bittersweet when they conclude a long campaign. You can save and load at any time, even in multiplayer, so you could have one campaign going for weeks or months if you like.