It has become second nature to see US borderlands as spaces of demarcation – with fences, walls, and patrols to mark continental divides. This is, however, a relatively recent view. We might trace its roots to 1910, when troops began patrolling the line following the outbreak of the Mexican Revolution; or to 1911, when the United States began to string up fence to keep tick-infested cattle in Mexico. Only by the 1920s and 1930s – with immigration quotas, a newly minted US Border Patrol, efforts to stem Prohibition-era flows of alcohol, and the repatriation of Mexicans in the Great Depression – did state-managed gate-keeping and partition become, in the words of historian Rachel St. John, “the defining feature of spatial organization along the border.”