In October 1913 Bierce, then aged 71, departed Washington, D.C., for a tour of his old Civil War battlefields. By December he had passed through Louisiana and Texas, crossing by way of El Paso into Mexico, which was in the throes of revolution. In Ciudad Juárez he joined Pancho Villa's army as an observer, and in that role he witnessed the Battle of Tierra Blanca.
Bierce is known to have accompanied Villa's army as far as the city of Chihuahua. His last known communication with the world was a letter he wrote there to Blanche Partington, a close friend, dated December 26, 1913. After closing this letter by saying, "As to me, I leave here tomorrow for an unknown destination," he vanished without a trace, his disappearance becoming one of the most famous in American literary history. Skeptic Joe Nickell argued that no letter had ever been found; all that existed was a notebook belonging to his secretary and companion, Carrie Christiansen, containing a rough summary of a purported letter and her statement that the originals had been destroyed.
Oral tradition in Sierra Mojada, Coahuila, documented by the priest James Lienert, states that Bierce was executed by firing squad in the town cemetery there. Again, Nickell finds this story to be rather incredible. He quotes Bierce's friend and biographer Walter Neale as saying that in 1913, Bierce had not ridden for quite some time, was suffering from serious asthma and had been severely critical of Pancho Villa. Neale concludes that it would have been highly unlikely for Bierce to have gone to Mexico and joined up with Villa.
All investigations into his fate have proven fruitless, and Nickell concedes that despite a lack of hard evidence that Bierce had gone to Mexico, there is also none that he had not. Therefore, despite an abundance of theories (including death by suicide), his end remains shrouded in mystery.