From 1793 to 1805, yellow fever devastated the port cities of the eastern United States. As the most pressing natural philosophical issue of the early national period, the search for the cause of the disease generated a heated debate that pitted “contagionists,” who thought the it was imported, against “localists,” who believed it arose from domestic sources. This article examines the fever investigators' uses of history to settle the controversy, thereby showcasing an unknown aspect of the early republicans' well-noted interest in the past. This "Thucydidean moment" (so named for their particular fascination with Thucydides' famous plague narrative) culminated in two massive books on the history of disease: the Treatise on the Plague and Yellow Fever by the contagionist James Tytler, and the Brief History of Epidemic and Pestilential Diseases by the localist Noah Webster. Whereas Tytler's work was roundly rejected by his philosophical peers, Webster's Brief History cogently situated the yellow fever epidemics in the sweep of history, and thus pushed the debate in favor of the localists. But the victory was a Pyrrhic one. The appeal to the past exposed problems that undermined investigators' hopes that history could serve as an empirical basis of disease inquiry. It also threatened early republicans’ pretensions to historical exceptionalism by forcing them to reckon with their own precarious places in the cycles of time.