The Cruel Stars by John Birmingham

With a ruthless sect of humanity intent on wiping out their species, five intrepid heroes must think like their enemies in this epic SF adventure.

They thought the Sturm were dead. They were wrong.

Centuries after their defeat, the enemy has returned with an overwhelming attack on the fringes of human space. On the brink of annihilation, humankind's only hope is a few brave souls who survived the initial onslaught: Commander Lucinda Hardy, commander of the Royal Armadalen Navy's only surviving warship; Booker3, a soldier of Earth, sentenced to die for treason; Alessia, a young royal forced to flee when her home planet is overrun and her entire family executed; Sephina L'trel, the leader of an outlaw band.

And, finally, retired Admiral Frazer McLennan, the infamous hero of the first war with the Sturm, who hopes to rout his old foes once and for all – or die trying.

These five flawed, reluctant heroes must band together to prevail against a relentless enemy and near-impossible odds. For if they fail, the future itself is doomed.

As new condition.