This question already has an answer here:
I am looking for the title or author of a atory I read back in the 90's, no idea when it was written. The plot goes something like this: Humanity has gained access to space travel and sends 9 or 12 survey ships towards galactic center, only 1 returns with stories of an enemy so rapacious that humanity starts preparing for war. After a long time, the enemy hasn't shown up and people begin to doubt the need to keep building for war. The government engineers a plan to turn the core systems against the fringe worlds and start warring amongst ourselves so that we may be prepared for when the real enemy shows up.
Written with the focus on one spaceship captain battling in our own internal war.