Among the world’s languages, Vietnamese provides unique insight into the cosmopolitan dynamism of premodern Asia.Modern notions of language history are often constrained by nationalist narratives, focused on bolstering a particular nation’s social, cultural, or political identities.A closer look at the Vietnamese language reveals a rich record of interaction and transformation that does not fit easily within modern nation-state lines or boundaries. By employing philological, textual, and comparative linguistic methodologies, John D.Phan uncovers the history of a Sinitic language rooted in the Red River Plain of northern Vietnam, which he calls “Annamese Middle Chinese.” The life and death of this language stimulated dramatic transformations in the speech of the region, ultimately giving rise to a new and alloyed language over the early centuries of the second millennium—Vietnamese. Drawing connections among linguistic, demographic, intellectual, and cultural realities over time, Phan traces the story of the emergence of Vietnamese within the broader context of a cosmopolitan East and Southeast Asia.Lost Tongues of the Red River demonstrates how language forms a surprisingly intimate record of human interaction—one with unique potential to enrich and expand our understanding of the distant past.