conflict broke out in northern Mexico, led by Madero, Pascual Orozco, and Pancho Villa, and with support from portions of the middle class, the peasantry, and organized labor, Díaz was forced out. In the Treaty of Ciudad Juárez, Díaz resigned and went into exile. New elections were held in 1911, and Madero was elected, taking office in November. Opposition to his regime then grew from both the conservatives, who saw him as too weak, and from former revolutionary fighters, who saw him as too conservative. In a period in February 1913, known as the Ten Tragic Days, Madero and his vice president Pino Suárez were forced to resign and then assassinated. The counter-revolutionary regime of General Victoriano Huerta came to power, backed by the United States and its ambassador Henry Lane Wilson, business interests, and supporters of the old order. Huerta remained in power until July 1914, when he was forced out by a coalition of different regional revolutionary forces, including the forces of Pancho Villa and Emiliano Zapata. Wealthy landowner Venustiano Carranza formed the "Constitutionalist" political faction, and with military forces under the leadership of Álvaro Obregón, played an important part in defeating Huerta. When the revolutionaries' attempt to reach political agreement failed, Mexico plunged into a civil war (1914–15). Carranza emerged as the victor in 1915, defeating the Villistas and forcing Zapata back to guerrilla warfare. Zapata was assassinated in 1919 by agents of President Carranza. Many scholars consider the promulgation of the Mexican Constitution of 1917 as the end point of the armed conflict. For the United States, the Revolution proved profoundly consequential, as the violence unleashed by the conflict led between 600,000 and 1,000,000 refugees to flee into the United States. Along the border, residents in American cities feared revolutionary fervor for land redistribution would inspire the poor and working classes in the U.S. to rise up against Anglo property ownership and domination. The Texas governor made a formal request for U.S. troops to guard the border and keep the revolutionary ideals out of the United States. The border was effectively turned into a militarized zone. Barbed wire, spotlights, tanks, machine guns, and airplanes were brought in to surveil Mexican residents and maintain "order" in the borderlands. Around sixty-five hundred U.S. troops were stationed in El Paso, where they were ordered to keep Villistas from entering the city and keep watch over the Mexican refugees and