Rykolien Immigrants

The Rykolien Immigrants books by Warren Dunn take place on Rykol, and in Earth's solar system. It is about a group of people who are kidnapped by the alien Vekorna, brought to Rykol and are caught in the planet's wars. When they return to Earth centuries in the future, where it is now called Terra, they find a dangerous situation that could destroy the planet.   Four centuries into the future, humanity braces for the return of the vekorna, who are intent on taking more humans to help in the next wars. The aging crews from the original missions search for any defenses to protect humanity from the vekorna, while facing growing xenophobia and reduced access to systems that they themselves designed.