Social and political revolution in Mexico, fought between 1911 and the 1920s. The first social revolution of the twentieth century, the Mexican Revolution was unleashed by the uneven distribution of wealth and land caused by distortions traceable to the dictatorship of Porfirio Díaz, and the legacy of the colonial period. Enormously destructive, some one million people lost their lives and many more fled both within the country and to the United States. Innumerable books have been written about the revolution and its many aspects, but briefly the strife started as a factional struggle arising after Díaz's ouster. The conservatives rallied behind Carranza, and a more liberal faction behind generals such as Alvaro Obregón. The more radical groups, Pancho Villa in the North, and Emiliano Zapata in the South, fought whoever controlled the central government. By the 1920s the group behind Obregón consolidated political power, but there was another wave of fighting called the Cristero rebellion led by fervent Catholics in Jalisco and Michoacán opposed to the anticlericalism of many of the revolutionary leaders. Mexican politics evolved a one-party system with a president rotating every six years. This party, the PRI or Party of the Institutionalized Revolution, has governed since the 1930s, although Mexico has developed a multi-party system in recent decades.