A Princess of Mars (1917) by Edgar Rive Burroughs
Sometime after the American Civil War, Virginia gentleman and soldier of fortune John Carter is prospecting for gold out west when he set upon by an Apache war party. Taking refuge in a cave, something inside frightens the Indians off and Carter is transported to the planet Mars by some mysterious agency. He is captured by the Tharks, the four-armed, gigantic green men of Mars. Carter's courage combined with his much stronger Earth muscles impress the warlike Tharks and Carter's victories in combat quickly raise him to the rank of Chieftain. When the Tharks capture the far more human and lovely Princess Dejah Thoris of Helium, Carter finds a new purpose in life and dedicates himself to her rescue and protection.
A Princess of Mars is Edgar Rice Burrough's other great series. Though less well known to the general public than his Tarzan stories, the Barsoom (after the native word for Mars) series has been a perennial favourite for almost a century. Those who have been raised on Star Trek and Star Wars may be put off at first by a science fiction adventure where the hero wields a rapier and rides a six-legged Thoat, but Burroughs has created a world perfectly suited to the needs of romance and adenture. Taking his cue from Percival Lowell's writings about Mars, Burrough's Barsoom is a dying world where the population is shrinking as the water evaporates into space and the atmosphere is kept by huge air generating plants. The great old cities long abandoned, new city sates have sprung up along the canals used to keep the southern regions fertile and hoards of green barbarians ride across the mossy bottoms of the ancient oceans. Despite and advanced technology, the inhabitants prefer to fight with swords instead of their radium pistols because combat is first about population control and only secondly about settling disputes.
It's also a world that suits John Carter; a chivalrous southern gentleman whom the world has passed by. On Barsoom, he finds foes to battle, wrongs to write, and a beautiful princess to woo and win. This he manages through a thrilling series of marches and counter-marches where the newly-arrived Virginian must battle his own ignorance of local languages and customs as well as the dastardly villains who oppose him.
The character of John Carter isn't as well done as Tarzan, but then Burroughs did have a world to describe as well while Tarzan kicked about in a Jungle on familiar planet Earth. Wasting a bit of effort on the odd thoat.