Weep Not for the Dead (Kindle version)
by Gina Burgess
Captain Clinton Kincaid is the first man to fly the Colber shuttle to Jastrow Station and back to Earth. His next mission is to Mars. Something goes terribly wrong. Suddenly, his mission is modified by the Crothans when he is transported 100 years into the future, and only he and a specially trained crew can save Earth from the dreaded Draumans. Kincaid and his crew's first mission on the Starmada ship Discovery began as a peace mission to establish an alliance between Croth and Earth, but it turns into much more: Saving all humanity from certain slavery.
Earth was visited by a peaceful species from Croth in the mid 21st century. Humans were so violent at the time they attacked the Crothans before asking about their intent. Crothans beat a retreat, deciding humans had some more growing up to do before a friendship/alliance could be formed. About 100 years pass, and the Draumans discover the ripe pickings of Earth and her colonies. The Draumans are well known to be pillagers and resource-strippers reducing any world they conquer into metallic death heaps. With the Drauman attack on Earth imminent, the Crothans decide to try again to establish an alliance. They implement their plan to use humans for the second attempt to initiate the Peace Mission.