Det här låter som " Pilgrimens väg " av Gordon R. Dickson.
When the giant, armored, alien Aalaag descend upon Earth with their impregnable defenses and irresistible weapons, the planet submits completely. Three years later, expert linguist Shane Evert, as one of the few humans able to produce an intelligible version of the alien speech, is translator, courier, and slave of the Aalaag head honcho, Lyt Ahn. On his official travels, Shane witnesses the execution of a fellow human. The incident moves him to a small show of defiance: he scratches on a nearby wall a line drawing of a pilgrim. Mere months later, to Shane's astonishment, the symbol has spread worldwide as a focus of human resistance. Then Shane tricks the Aalaag into releasing attractive resistance-worker Maria; her associates persuade Shane to help them, since only Shane knows how the Aalaag think (they regard humans as their cattle). He concocts a dubious plan to force the Aalaag to leave.
Eventually, the plan goes into effect: organized, unarmed mobs of people dressed in pilgrim robes, defying death, hurl themselves bodily against the Aalaag strongholds. So the Aalaag, convinced that their human cattle can never be civilized, leave in disgust.
