(Mild spoilers for the 1998 Disney animated film below.)
Both films are based on the Chinese legend, "The Ballad of Hua Mulan," telling the story of a young woman in the Northern Wei era (spanning 386-536 CE) who takes her father's place when each family is required to provide one male to serve in the emperor's army. In this version, Hua Mulan is already a well-trained fighter—the Arya Stark of Chinese legend—and she serves for 12 years with none of her fellow soldiers ever suspecting her true gender.
Disney's 1998 animated film broadly follows the traditional storyline, except Mulan is not well-trained when she first runs away. The film also added a love interest, and a catchy original soundtrack. Mulan was released to critical acclaim, grossing $304 million worldwide and earning Golden Globe and Oscar nominations. That was a marked improvement on Disney's prior two animated films, The Hunchback of Notre Dame and Hercules, but well below the box office success of classics like Beauty and the Beast, Aladdin, and The Lion King. But it was good enough to merit a direct-to-video sequel in 2004, and a spot on the roster of the studio's ongoing live action remakes.