A Princess of Mars is, on the surface, a science-fiction adventure story that bears a closer resemblance to fantasy set in space than to typical science fiction fare, which generally has a heavier emphasis on the inner workings of the science involved. In fact, many elements of this novel adhere more closely to fantasy than to science fiction, not the least of which is the division of a single planet between several races and subcategories of races, all of which are at odds with one another, drawing parallels to such masterpieces of fantasy as The Lord of the Rings and Earthsea. Regardless, this is a fast-paced, entertaining novel with a heavy dose of romance and a decided fixture upon the imagination.
Many critics have compared this story to a classic Western about the American Frontier. There are certainly many similarities: crossing a vast, deserted land to find a more primitive people group with differently colored skin and a distinct, technologically lacking way of life. There is also the characteristic wanderlust, a desire to reach uncharted lands and explore the depths of untainted beauty. And, of course, the introductory chapters actually do take place on the Western frontier, at a gold mine in Arizona. This coincidence makes the reader wonder if this connection was one intended to be drawn all along.
It's also important to note the situation of Edgar Rice Burroughs as he was composing this novel. He was struggling to stay afloat in the world of business - he had yet to make a name for himself, and the money wasn't flowing in fast enough to take care of his family. He was feeling trapped, most likely dreaming of beautiful open spaces and the glories of freedom. To some extent, A Princess of Mars can be seen as Burroughs's own escape from his stressful reality; hounded by the Apaches (like Burroughs was hounded by creditors), Carter accidentally stumbles upon a whole new world that provides for him an adventure, glory, honor, and a perfect family. It certainly seems like a fantasy on the part of Burroughs, who likely longed for such things to happen to him, although this is entirely speculation.
