Here's the plot from the back cover:
The first human intersellar civilization was destroyed by the Mizari, alien Sterilizers from a black star in the Ursa Major cluster.
Sixty-thousand years later, humanity's far flung remnants have reunited to form a new galactic empire. The Unified Worlds are vibrant, restless, outward bound—and haunted by the ever-present threat of the shadowy Mizari. A system of Sentinels and Shields monitors the frontier, while mighty Defender ships protect the home worlds. But are they enough to counter the Sterilizers?
Defense Director Wells thinks not. He spearheads the development of the world-wrecking Triad weapon to battle the enemy. Chancellor Janell Sujata opposes his plan, and pits herself against the Nines, the elitist secret society of which Wells is a leading member.
But visionary Merritt Thackery will decide the fate of both races, alien and human. The aged explorer races his stolen ship toward the Mizari system, hurling into the unknown risks of imminent contact—or intergalactic extermination…
That pretty much sums it up. I read this when I was 18 or 19. Now, it seems much less well done. Hamhanded even. I prefer my anti-military treatises to be a bit less ham-handed.