The Apocalypse Troll by David Weber. Short summary

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The raison d’être of the Kang civilization is the destruction of every intelligent life form in the universe. However, the only ones who fight back against the destroyers is humanity.

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For many who live on the planets of various technologically and culturally advanced nations, the word «Kangs» is a nightmare. Because it means a vast group of aliens, driven only by one goal: to destroy. For the Kangs, every intelligent life form is an irreconcilable enemy, subject to the destruction of its every representative and everything that this civilization has created at all.

Few could give a decent response to the Kangs. However, when the humanoids decided to subject humanity to destruction, they met with an incredible response. The human space fleet succeeded in defending the conquests of their ancestors against the murderers and executioners. After several centuries of struggle with the aliens, the people managed to push the aggressors to the outskirts of the universe and put their race on the brink of ruin and destruction.

To prevent their defeat, the remnants of the Kangs send a small detachment to the days when humans have not yet entered space.