This novel, written in 1913, concludes the initial saga of John Carter on Barsoom. The first three MARS novels, very similar to Tolkien's Ring trilogy, are really one story, interrupted by cliffhangers at the end of each novel.
Bear in mind that Ed was writing for his primary market of the day, the pulp magazines. Novels were serialized over 6 or so issues and required cliffhangers to bring the customers back for the next installment. After the magazine run, the novels were often serialized in local newpapers nationwide. Cliffhangers were the engine driving the business of popular fiction.
In the time since he completed The Gods of Mars, Ed had created yet another of his amazing series - the Inner World, aka Pellucidar. The Warlord of Mars was Ed's ninth book written since his first one in 1911. He maintained this feverish amount of work well into the 1940s.
Warlord picks up only 6 months after the conclusion of The Gods of Mars. John Carter's quest to free Dejah Thoris takes him literally from pole to pole on Barsoom.
Before the final happy ending, we meet new races and new Barsoomian creatures. How apt!