Three worlds to conquer — Poul Anderson

Jupiter- where no human could live-- but where men had strange allies... and stranger enemies!
Ganymede-where human settlers lay helpless under the guns of a spaceship directed by a madman!
Earth-freed from a planet-wide tyranny, but facing total destruction from space!
The destinies of these worlds were strangely linked--and in the hands of a man sentenced to instant death!