States and empires have long struggled to exert meaningful control in the arid and mountainous parts of today's Mexican north and U.S. Southwest, an area I call the North American Sierra. My research, focused on Hispano settlers, independent Indians, and Anglo exiles occupying the area in the eighteenth and nineteenth centuries, explores a singular region that continues to be subsumed by the histories and borders of three distinct manifestations of state and empire. Pushing against disciplinary boundaries that typically cleave the region's history along colonial-national and Latin American-U.S. lines, I ask what kinds of spaces pre-existed, co-existed, and potentially undermined the "official" spaces of northern New Spain, independent Mexico, and the United States. How did the people (and peoples) that lived in these spaces conceive of themselves and one another? How did they conceive of and experience place? Addressing these questions requires examining the relationship of various polities to each other but also to local ecology and topography, which, in this arid and mountainous region, both tested and aided them. My working hypothesis is as follows: In the North American Sierra, small autonomous polities were best suited to a harsh ecological and contested geo-political environment. Communal autonomy was thus a shared value and strategy, rooted in the relationship of political structure to ecology, that came to constitute a cross-cultural regional character. My project historically reconstructs this region – the North American Sierra – as the product of both externally imposed state-level action and more nuanced cultural, political, and spatial historical realities.