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 transnationa.