Star Trek: Gateways, book 06 : Cold Wars
Peter David2001

Missing for two hundred millennia, the legendary Iconians have returned, bringing with them the secret of interdimensional teleportation across vast interstellar distances. Awakened once more, their ancient Gateways are rewriting the map of the galaxy, and nowhere more than in the New Frontier....

A century ago, the imperial Thallonians separated two feuding alien races, depositing each of them on a new world safely distant from that of their ancestral enemies. Now, however, the Gateways have made it possible for the long dormant blood feud to begin anew. Captain Mackenzie Calhoun of the USS Excalibur and his partner, Captain Elizabeth Shelby of the USS Trident, find themselves fighting a losing battle to keep the horrific violence from escalating, even as they gradually realize the catastrophic danger posed by the Gateways themselves!