My book project, The Biosphere in Postwar America: The Politics of Planetary Knowledge, is a history of the idea of the planetary environment as a steady-state system, told through the history of the idea of the biosphere in the postwar United States. First theorized in the 1920s by Russian mineralogist Vladimir Vernadsky, the biosphere casts the whole of planetary matter as an integrated system of chemical exchange, framing the interactions between organisms and environments in terms of the cyclical flow of chemicals between the two domains. When the concept was introduced to U.S.-based scientists in the 1940s, however, it coincided with a broader scientific discourse on systems and was forever changed as a result. In the American environmental sciences, not only did the biosphere connote a system of cyclical exchanges, but these cycles were understood to exist in a state of steady-state stability. The book follows the biosphere as it took shape as an object of knowledge across different experimental contexts to show how the experimental techniques developed for studying the concept not only produced new environmental knowledge, but also recursively reinforced the very ecological theory upon which they were founded—the idea of the planetary environment as a steady-state system. At the same time, the project scales from the level of laboratories to the level of broader, systems-related historical contexts to show how knowledge about the biosphere was produced and reified through environmental scientists’ encounters with a broad miscellany of domains of geopolitics. Ultimately, the project uses the history of the biosphere to argue that the concepts we use to scientifically theorize the natural world—especially those anchored in the logic of systems—are built on Cold War formations of power, including Cold War militarism, the statecraft of natural resource security, and the Space Race-era quest for extraterrestrial territorial conquest.