Corewar may be the first example of the game in its purest form, but earlier than that was Robotwar (first on the PLATO system in the 1970s then on the Apple ][ in 1981) which involved players writing simple programs to control robots in a BASIC like language. You could either compete against a friend or compete against a series of programs included with the game.

https://en.wikipedia.org/wiki/RobotWar