Since the 1950s, most Korean scholars have believed that Paekche King Kŭnch'ogo (r. a.d. 346–375) expanded his territory southward from his base in the Han River basin (present day Seoul) to encompass the entire southwestern Korean peninsula in a.d. 369. Although the nature of his southern conquest has been debated by scholars, the conquest itself has rarely been questioned. Upon closer inspection, however, we find that the sole historical source, the Nihon shoki (compiled a.d. 720), does not describe a Paekche conquest but a punitive military expedition from the Japanese archipelago. This account was later used to justify Imperial Japan's annexation of Korea in 1910. This article explores the twisted historiographical journey of this ambiguous passage from an early Japanese hagiography and its transformation from a tool of Japanese imperialism into an unquestioned Korean narrative of Paekche territorial expansion. A critical examination of the original text and the archaeological context of this supposed Paekche invasion into the southwestern Korean peninsula suggests that such a dramatic southern Paekche expansion was unlikely to have taken place as early as the late fourth century a.d.