Ecological imperialism refers to the center’s robbery of nature from the periphery, emphasizing the ecological dimension of trade, often overshadowed by monetary concerns. Examining historical cases like the nineteenth-century guano trade between Peru and Europe reveals this dynamic, wherein the involvement of the United States has been often overlooked. Utilizing archival data from British, French, Peruvian, and U.S. sources, I demonstrate how guano served as an ecological foundation for U.S. expansion overseas starting in the 1850s. This includes the significance of guano islands as the first U.S. overseas territories, considerations of annexing South American islands for guano, and the role of guano in partly inspiring the construction of the Panama Canal. Overall, the guano trade exemplifies an imperial-ecological process that reshaped global legal, geopolitical, and trade systems. Additionally, I discuss the contemporary relevance of this history, particularly concerning lithium, emphasizing the importance of prioritizing ecology in center-periphery analyses.