Supplicants - The Canta Libre Trilogy, #2

First contact with an intelligent alien species had not just been a disaster, it had been a bloodbath. Now, what is left of the crew of the Canta Libre, Earth's emissaries, must travel on, deeper into the galaxy, to plead for help from another, more powerful race, the Iffot Onni. But their would-be saviours are in the throes of a violent civil war when the humans arrive and Iffot leaders are in no mood to deal with backward aliens from the Milky Way's rimward wasteland.
Struggling to bring their message to the Iffot kings despite factional wars and interspecies treachery, the humans hear that a war has broken out at home. Now their own people have cancelled the mission and are demanding the crew of the Canta Libre return to Earth to face trial for treason. But Captain Robert Ashton and his unlikely collection of civilians, scientists and engineers are determined to save humanity, whatever the cost.