The Enemy of My Enemy. The Cull Chronicles, #2

When Earth is conquered by a technologically superior race called the Grath, humanity is forced to work--and fight--for them. But not all humans are content to submit to their masters... Jason Cull has never really imagined he'd carry the title of captain. But when the resistance movement Freedom's launch is botched and their leader is killed, Jason ends up the man in charge. Since the Grath conquered the Earth and started putting humans to work, Jason has struggled to believe he's special despite being one of the select few chosen to help fight battles in place of humanity's new masters.
Being kidnapped by the resistance movement and made their leader has put him in the uncomfortable position of being the one to figure out how to literally save the world. His only consolation is that the Freedom's former leader at least left him with a vague idea what to do next: Find the alien race, the Bettarians, who are already at war with the Grath and strike a deal with them to help win back Earth.
The Bettarian's empire may be rumoured to be vast, but Jason's seemingly fruitless ferreting from one end of the galaxy to the other has him concluding the universe is one pretty enormous haystack. Beyond that, the mission to scour outer space for allies gives Jason lots of time to mull over the former leader's ill-conceived plan. He's left with a question he has no answer to by the time his search is successful: What can one ship full of refugees from a backwards planet possibly offer a sweeping and ancient alien empire in exchange for their assistance? Especially when it turns out that they all have much bigger problems than the Grath...