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In the book, humans are allowed to travel in a race of aliens spacecraft from planet to planet. The aliens won't tell them the secrets of space travel. You find out that this is because interstellar ships can be sent into stars to destroy whole solar systems.
They find this ship on a frozen planet. They find out that their alien "friends" are at war with another species. The friendly aliens are water creatures or something. Some of them can tell the future or something like that.
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.
