The Blackcollar — Timothy Zahn

Decades after a successful invasion of Earth and the Terran Democratic Empire by the Ryqril—hostile, leathery-skinned aliens—resistance fighter Allen Caine is training for an undercover mission. He will assume the identity of an aide to the senate—part of the government that colludes with the invaders. But when the mission begins earlier than planned, Caine finds himself stuck on the off-planet outpost of Plinry with no idea of what awaits. He’s responsible for the most important mission undertaken by the resistance in twenty years, and when the operation goes awry, Caine’s only hope is to locate Plinry’s so-called blackcollars—the elusive, martial arts–trained guerilla force whose wartime resistance efforts are legendary. With his life and the freedom of everyone in the TDE on the line, Caine’s success will depend on whether or not he can find them. . . .