Star Trek: Gateways, book 02 : Chainmail
Diane Carey2001

Dangerous remnants of an extinct interstellar civilization, the Gateways connect the Alpha Quadrant with the farthest reaches of the galaxy. Hidden away in various corners of the universe, the ancient portals could be the future of space travel, but they may also provide a open doorway for an invasion from beyond!

Twenty years ago, in the space near Belle Terre, a caravan of alien vessels disappeared into a gigantic Gateway. Now the descendants of those aliens have returned, armed with incredible new weapons and abilities. Captain Nick Keller of the U.S.S. Challenger, already struggling to maintain peace in the troubled sector, must now cope with a fleet of hostile aliens driven by their own fanatical agenda!