Political awareness of the tensions in U.S.—Mexico relations is rising in the twenty—first century; the American history of its treatment of illegal immigrants represents a massive failure of the promises of the American dream. This is the untold history of the United States Border Patrol from its beginnings in 1924 as a small peripheral outfit to its emergence as a large professional police force that continuously draws intense scrutiny and denunciations from political activism groups. To tell this story, MacArthur "Genius" Fellow Kelly Lytle Hernández dug through a gold mine of lost and unseen records and bits of biography stored in garages, closets, an abandoned factory, and in U.S. and Mexican archives. Focusing on the daily challenges of policing the Mexican border and bringing to light unexpected partners and forgotten dynamics, Migra! reveals how the U.S. Border Patrol translated the mandate for comprehensive migration control into a project of policing immigrants and undocumented “aliens” in the U.S.—Mexico borderlands.