Starbound — Joe Haldeman

Carmen Dula and her husband have spent six years travelling to a distant solar system that is home to the enigmatic, powerful race known as "The Others," in the hopes of finding enough common purpose between their species to forge a delicate truce.

By the time Carmen and her party return, fifty years have been consumed by relativity-and the Earthlings have not been idle, building a massive flotilla of warships to defend Earth against The Others. But The Others have more power than any could imagine-and they will brook no insolence from the upstart human race.