In Conquest Born by C. S. Friedman (1991)

Sometimes a race of humans will make war just to have war. This intergallactic adventure is quite epic. It explores two races of humanity that have warred forever for reasons unknown. There are spies and assasins and war strategies. Space travel and battle are explored, but it really spends more time on people's behaviors and feelings and the nuances of psyche and manipulation. It is sort of a Battlestar Galactica but with two equal races of beings. It then ties back in to history and has a stunning conclusion. Very well done for a first book by the author C.S. Friedman. I especially enjoyed her introduction.