When rebels captured the border city of Juárez, Mexico, in May 1911 and forced the abdication of President Porfirio Díaz shortly thereafter, they not only overthrew the western hemisphere’s oldest regime but also inaugurated the first social revolution of the 20th century. Driven by disenchantment with an authoritarian regime that catered to foreign investment, labor exploitation, and landlessness, revolutionaries dislodged Díaz’s regime, crushed an effort to resurrect it, and then spent the rest of the decade fighting one another for control of the nation. This struggle, recognized ever since as foundational for Mexican politics and identity, also had enormous consequences for the ethnic makeup, border policing, and foreign policy of the United States. Over a million Mexicans fled north during the 1910s, perhaps tripling the country’s Mexican-descent population, most visibly in places such as Los Angeles that had become overwhelmingly Anglo-American. US forces occupied Mexican territory twice, nearly bringing the two nations to outright warfare for the first time since the US–Mexican War of 1846–1848. Moreover, revolutionary violence and radicalism transformed the ways that much of the American population and its government perceived their border with Mexico, providing a rationale for a much more highly policed border and for the increasingly brutal treatment of Mexican-descent people in the United States. The Mexican Revolution was a turning point for Mexico, the United States, and their shared border, and for all who crossed it.