At dozens of internal checkpoints across the country, Border Patrol agents stop and question passing motorists on their citizenship. Elsewhere, officers engage in roving traffic stops aimed at interdicting illegal immigration inside the United States. Agents at checkpoints require neither a warrant nor individualized suspicion to stop passing motorists and inquire about the occupants’ citizenship, or to inspect private lands within twenty-five miles of the border. Taken together, these “defense in depth” measures amount to an extraordinarily expansive law enforcement effort carried deep into the U.S. interior.