Line in the Sand details the dramatic transformation of the western U.S.-Mexico border from its creation at the end of the Mexican-American War in 1848 to the emergence of the modern boundary line in the first decades of the twentieth century. In this sweeping narrative, Rachel St. John explores how this boundary changed from a mere line on a map to a clearly marked and heavily regulated divide between the United States and Mexico. Focusing on the desert border to the west of the Rio Grande, this book explains the origins of the modern border and places the line at the center of a transnational history of expanding capitalism and state power in the late nineteenth and early twentieth centuries. Moving across local, regional, and national scales, St. John shows how government officials, Native American raiders, ranchers, railroad builders, miners, investors, immigrants, and smugglers contributed to the rise of state power on the border and developed strategies to navigate the increasingly regulated landscape. Over the border's history, the U.S. and Mexican states gradually developed an expanding array of official laws, ad hoc arrangements, government agents, and physical barriers that did not close the line, but made it a flexible barrier that restricted the movement of some people, goods, and animals without impeding others. By the 1930s, their efforts had created the foundations of the modern border control apparatus. Drawing on extensive research in U.S. and Mexican archives, Line in the Sand weaves together a transnational history of how an undistinguished strip of land became the significant and symbolic space of state power and national definition that we know today.