To extend from timbp's comment above, linking to his prior answer at Can't remember sci-fi novel with loner female cargo pilot carrying contraband technology, this may be Gary Gibson's Stealing Light.
For a quarter of a million years, an alien race has been hiding a vast and terrible secret
In the 25th century, only the Shoal possess the secret of faster-than-light travel (FTL), giving them absolute control over all trade and exploration throughout the galaxy. Mankind has operated within their influence for two centuries, establishing a dozen human colony worlds scattered along Shoal trade routes. Dakota Merrick, while serving as a military pilot, has witnessed atrocities for which this alien race is responsible. Now piloting a civilian cargo ship, she is currently ferrying an exploration team to a star system containing a derelict starship. From its wreckage, her passengers hope to salvage a functioning FTL drive of mysteriously non-Shoal origin. But the Shoal are not yet ready to relinquish their monopoly over a technology they acquired through ancient genocide.
This review for Nova War, one of the sequels, notes the war aspect:
It becomes rapidly clear to them, that the humanity’s limited knowledge of the rest of the galaxy – filtered through the Shoal – is direly inaccurate. The Shoal have been fighting a frontier war with a rival species, the Emissaries, with their own FTL technology for over fifteen thousand years.