I read this about 2003, in a library book so it's hard to say how old it was. The 1990s at the latest.
There were two advanced space travellers - they may have been scientists or salesmen, or both. They were bored and as a form of entertainment they gave or sold two opposing planets advanced (most likely nuclear) weapons to see how long it would take for them to obliterate each other/themselves.
They were acting alone, not for any governments.
There was a line from one to the other like:
Shall we play war?
Shall we play extinction?
Let's have a game of extinction/war/obliteration
Something along those lines.
I never actually got to the finish so no info on conclusion.