Enforcement: The Undeclared Ja-gee War: Book 1 of the Laner Series — Paul Arthurs

Ten years ago, starship captain Arthur Laner resigned from Enforcement, started a salvage business, and became a pirate.
Now, hundreds of light years from Earth, a Human exploration vessel and a Ja-gee warship meet for the first time. The results are disastrous, and the Interstellar Concordance is unprepared for the coming war with the Ja-gee Empire.
Laner is dragged back into Enforcement, even as his former comrades confront the Ja-gee and perish amidst the debris of their shattered starships.
Given a green crew and an unproven starship, Laner must bury his past and race to the Front, ever closer to confrontation with a seemingly unstoppable foe.