Timothy Zahn 
Backlash Mission [PDF ebook] 

In the sequel to Blackcollar from the #1 New York Timesbestselling author of Star Wars: Thrawn, Allen Caine is back on the front lines of an alien war. Denver, Earth. The twenty-fifth century. After a devastating alien invasion, the Terran Democratic Empire is occupied by the Ryqril race. The once-heroic resistance warriors known as the blackcollars now serve as strong-arm security for Denver’s criminal elements. When Allen Caine completes his year-long blackcollar training on the planet Plinry, he and his elite team head to Earth to strike out against the Ryqril puppet government. But there’s no way of knowing whether the remaining blackcollars in Denver will be with him, or against him. . . .