Bohemia Interactive released the source code of the first Arma game, at the time known as Operation Flashpoint: Cold War Crisis. Licensed under GPLv3, except third party components.

https://wikipedia.org/wiki/Operation_Flashpoint:_Cold_War_Cr...

The game features a large open world, multiplayer support, a full mission editor with support for scripting via an in-house language known as SQS, as well as comprehensive modding capabilities.

https://community.bistudio.com/wiki/SQS_Syntax

The codebase has been modernized to C++20.