Battlespace (The Legacy Trilogy, Book 2) by Ian. Douglas ISBN 13: 9780380818259

When called to do battle many light years from home, the 1st Marine Interstellar Expeditionary Unit rose to the challenge -- and now thousands of enslaved humans have been freed from the alien yoke. But Earth is twenty-one years older than the home planet they originally left, and the Marines need time to retrain and readjust -- time they do not have, due to the bizzare disappearance of a detachment of their brothers- and sisters-in-arms. It is a mystery, but there is a starting point: an ancient wormhole threading through the Sirius system. Whatever waits on the other side must be confronted, with stealth, with force, and without fear -- be it an ancient enemy or a devastating new threat. The Marines are heading into the perilous unknown . . . and what transpires there could reshape the universe for millennia to come.