Because the Ballad of Mulan is an anonymous and undated work, very little can be said about it with certainty. It was most likely composed during the era of Northern Wei and passed down orally for over a century until it was finally preserved in written form during the Tang Dynasty (source http://bit.ly/2KizXBx).
The version which was finally put into writing begins with Mulan sitting in front of her loom, weeping over the fact that her father has just received his conscription orders. Because Mulan has no elder brother, she is resolved to take her father’s place. Upon returning, she is offered a prominent position but turns it down; her only request is that she be sent home to her family.
After Mulan is reunited with her parents and she has resumed her feminine appearance, her comrades are flabbergasted to discover that she is a woman—for the twelve years that they fought together, none of them had suspected anything. The ballad concludes by commenting, “When a pair of rabbits run side by side, who can distinguish male from female?”
This final line is a direct challenge against the ancient proverb “Men are wolves who fear weakness; women are rats who fear strength” (生男如狼，猶恐其?；生女如鼠，猶恐其虎). The thought that men and women are nearly identical would have been considered outrageously progressive.
Although most modern historians assume that the Ballad of Mulan is a work of fiction, it was not until recent times that the historicity of Mulan began to be questioned. Prior to the twentieth century, the story of Mulan was assumed to be a real historical event. A memorial dedicated to Mulan during the Yuan Dynasty explicitly mentions the Ballad of Mulan and cites it as a historical document. During the Ming Dynasty, a number of historians became increasingly interested in Mulan’s story (see The Legend of Mulan During the Ming Dynasty.
Nevertheless, the arguments which modern historians make when casting doubt upon the historicity of Mulan’s story are weighty. The historical documents which insist that Mulan was a real person propose contradictory accounts of her life. Each historian proposes a different set of details about her family, the time in which she lived, and the enemy against whom she fought. Because no one historical account has emerged as the authoritative record of her life, the majority of modern historians have decided to reject all of these so-called historical documents and conclude that Mulan’s story has no true historical basis.