The Silent Invaders / Battle on Venus

Abner Harris was sent to Earth on a mission of extreme urgency: the universe was in danger of enslavement by the pebble-skinned Medlins, and the fight against them called for Harris to assume the disguise of a flesh-and-blood Earthman. But once in that new synthetic body, he discovered that the real villains of space were not the Medlins or the people of Earth; THEY WERE HIS OWN KIND. Suddenly he was alone, alienated from his own race, hated by the Medlins, and an imposter on Earth. No matter what side he chose he'd be a traitor. Yet choose he must... or forever remain a man without a planet. Earth's first spaceship to Venus landed amidst a war where strange weapons like the archaic ones used in the old wars on Earth in the twentieth century hurled shells at each other. But this war had lasted over a thousand years—and by remote control! George Starkey had to find a way to stop the war before the little group of astronauts became early casualties. But how? Where were the headquarters of the contending sides and how do you tell a robot tank that you're neutral? But George had an ally, a Venusian girl who thought stealing was virtuous—and, unknowingly, he had something else that turned out to be the most valuable substance on Venus—a box of chocolate bars!