This article highlights how Americans used intertwined arguments about space and geography to justify and denounce different territorial configurations from the late eighteenth century through the Civil War. These arguments wove together ideas about geography (a set of physical, topographical features) and space (the human constructs that shape movement and human relations) in everything from theoretical arguments about the ideal size of republics to specific ideas about how rivers, mountains, oceans, and other features related to the proper shape of the nation. Americans evoked a variety of assumptions about how the physical landscape shaped human activity. They also made arguments about space and the ways that places were physically, and thus should be politically, connected. Highlighting an underappreciated current of manifest disunion, this article illustrates how different factions used geographic and spatial arguments not only to support and condemn varied expansionist visions, but also to justify disunion and secession.