The game is 2300 AD (earlier known as Traveller: 2300), which was GDW's sequel to the World War 3 RPG Twilight 2000. The KeferKafer have a similar biochemical response to humans when faced with danger and/or stress, however the hormone that's analogous to human adrenaline doesn't improve their physical capabilities, it improves their intellectual ones, and they retain a slight increase to intelligence afterward that gradually becomes accumulative, meaning that Kafers not only become dramatically smarter during battle, after the battle while their intelligence decreases again, it doesn't decrease as much. Therefore, veterans of repeated encounters will end up being just as smart as humans right from the start.
Because, like humans, even observing a conflict could cause a sympathetic physical reaction, the culture that eventually emerged focused on ritual violence: those participating would get smarter, but the people watching would likewise get a boost, albeit a smaller one, that was sufficient to maintain a technologically advanced society. The ruling classes specifically arranged things to ensure that they would regularly get that boost to stay intelligent, and thus in charge.