On March 10th, we celebrate "Three Flags Day," marking the finalization of the Louisiana Purchase. Peter Kastor, Professor of History and American Culture Studies at Wash U, joins Nate Gatter to discuss the historical significance of this pivotal event. Kastor dives into the 1804 ceremony that transferred Upper Louisiana from Spanish to French to American control and its broader impact, including its connection to the Lewis and Clark expedition. He also explores the historical context of St. Louis, including the role it played in the American Revolution and its growth in the early 1800s. This episode sheds light on regional history and the complexities of early American expansion, providing valuable insights into Three Flags Day and St. Louis’ role in American history.