There are some amazing open source games available and Warzone 2100 is definitely one of them. It was originally developed by Pumpkin Studios and published by Eidos Interactive. It was originally released in 1999 on PC and PlayStation.
In late 2004 the source code and most of the data was released and a few years later the rest of it. It is now also available for Windows 7-10, macOS, FreeBSD, AmigaOS, AROS, MorphOS, Linux, NetBSD and OpenBSD. Since then the Warzone 2100 Project has been worked on.
According to the GitHub page “Command the forces of The Project in a battle to rebuild the world after mankind has been nearly destroyed by nuclear missiles. “. You can play it in single-player mode or against other players on a LAN or on the internet.
To get a feel for the game you can view the online guide.
Once you’ve played it, you could consider contributing to the project. It’s an open project and the issue tracker currently shows 213 open issues.