Translated and edited by Chris Baker and Pasuk Phongpaichit Yuan Phai, the Defeat of Lanna, may be the oldest work of literature from Siam, most likely written after a battle fought in approximately 1475. The poem recounts many details of fifteenth-century events not found elsewhere. It has the earliest and most detailed description of a Siamese army, the most elaborate eulogy of an early Thai monarch, and a fascinating discussion of the concept of loyalty. The scenes of personal treachery, quiet heroism, bloody combat, and looting after victory give an absorbing image of early Siam and its ethos. This translation by a prize-winning team shares a remarkable but little-known work with a wider audience. It is fully annotated to explain many obscure words and concepts, and it is rendered in blank verse to convey the rhythm and atmosphere of the original. The introduction and afterword provide historical background, analyze the poem’s form and content, and discuss its literary and historical significance. This publication was made possible in part by a translation subvention from the Henry Luce Foundation.