Warzone 2100 open source 3D real time strategy game
Warzone 2100 is a free and open source 3D real time strategy (RTS) game for Windows, macOS, Linux and FreeBSD. It comes with single player campaign, LAN / Internet multiplayer, play versus AI bots and more than 400 different tech tree. It is released under the GPLv2+ license. You need at least 1 GB RAM, OpenGL 2.1+ support and 150MB storage space to run this game.