Enemy territory - Keith R. A. DeCandido

For centuries, the Elabrej believed that they were alone in the universe, and that no sentient life existed outside their home star system. But their certainty is shattered when a controversial exploration vessel of their own making encounters _" and fires upon _" an alien ship. The aliens return fire and destroy them _" and then come to Elabrej to investigate.
Meanwhile the crew of the I.K.S. Gorkon, exploring the uncharted Kavrot sector, learn that their brother ship, the I.K.S. Kravokh, was attacked by an unknown alien vessel and subsequently destroyed it. After setting a course to investigate this new people, the Kravokh disappears _" but a massive alien fleet is gathering at its last known location. Captain Klag must find out what happened to the Kravokh, and who this unknown foe of the Empire may be