Swords of Mars (Barsoom Series by Edgar Rice Burroughs

Disguised as a mercenary warrior, John Carter sought to break the power of the Assassins of Zodanga. Spying on their councils, the Warlord discovered a plot to kidnap his beloved Dejah Thoris. But it was too late to save her. She was already in space, on the way to Thuria, Barsoom's nearer moon!