"You are outgunned. You are massively outnumbered. You must win." These are your orders. 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. You'll be wanting the demo to really know what we mean, there's nothing else on the market remotely like this (as many reviewers have glowingly pointed out).
Simply put got this and all it's expansions from Steam and it is by far the largest and most strategic RTS I've played. It also has an AI that puts most of the games industry to shame a fantastic game and for the current price a steal when you consider the content and variety that is present. Fair warning this game has a steep learning curve especially if you approach it like a normal RTS.