Why does contentious history play such an outsized role in some international relationships? Why do these “history wars” endure, overriding incentives to reconcile? Despite their demonstrable importance, history wars have generally been neglected by conventional conflict and security literature; and, while scholarship concerning the international politics of memory has expanded significantly, overarching frameworks addressing these questions remain underdeveloped. In this article, drawing on theories of memory politics, relational identity and ontological security, I analyze history wars as mnemonic encounters: sites at which national identities are constructed in relation to one another through remembering and forgetting shared history. Within such encounters, history wars may arise and persist where each side's mnemonic practices involve conflicting, negative representations of the other, and such representations constitute an important element of their national identities. This occurs because the rearticulation of conflictual representations constitutes both a means by which the national community is reproduced and a defense mechanism against the ontological threat posed by the other side's counter-constructions. Illustrating this framework, I explicate the construction and persistence of Japan and South Korea's “history problem,” drawing on extensive fieldwork and a discourse analysis of over one thousand original-language texts from both countries across politics, media and culture.