Warzone 2100

In Warzone 2100, you command the forces of The Project in a battle to rebuild the world after mankind has almost been destroyed by nuclear missiles. The game offers campaign, multi-player, and single-player skirmish modes. An extensive tech tree with over 400 different technologies, combined with the unit design system, allows for a wide variety of possible units and tactics. Warzone 2100 was originally developed as a commercial game by Pumpkin Studios and published in 1999, and was released as open source by them in 2004, for the community to continue working on it.

Note: the Steam version does not have a Linux build up and is not actually supported by anyone. Use the direct-download open source version linked.