Mexican American history, or the history of American residents of Mexican descent, largely begins after the annexation of Northern Mexico in 1848, when the nearly 80,000 Mexican citizens of California, Nevada, Utah, Arizona, Colorado, and New Mexico became U.S. citizens. Large-scale migration increased the U.S.' Mexican population during the 1910s, as refugees fled the economic devastation and violence of Mexico's high-casualty revolution and civil war. Until the mid-20th century, most Mexican Americans lived within a few hundred miles of the border, although some resettled along rail lines from the Southwest into the Midwest. In the second half of the 20th century, Mexican Americans diffused throughout the U.S., especially into the Midwest and Southeast, though the groups' largest population centers remain in California and Texas. During this period, Mexican-Americans campaigned for voting rights, educational and employment equity, ethnic equality, and economic and social advancement. Spanish period Spanish entry into what is now the United States southwest began in 1540, when Francisco Vasquez de Coronado, his 230 Spanish soldiers, 800 Indigenous Mexicans, and three women marched into the Rio Grande valley. Soon after, Juan Rodriguez Cabrillo led the first expedition into Alta California in 1542, when he landed in modern-day Santa Barbara. There are reports that the explorer Marcos de Niza entered Arizona in 1539, but scholars have cast doubt on his fabled exploration for the Seven Cities of Gold. Full-scale Spanish colonization of the Southwest did not begin until 1598, when the Spanish government, under pressure from the Catholic Church to Christianize the Coahuiltecan peoples of the Rio Grande Valley, selected Juan Oñate to cross the Rio Grande and establish a permanent settlement in San Juan Pueblo, near present-day Espanola. Rumors of hidden gold and silver ores in New Mexico circulated in the lead-up to the Oñate settlement, but none were ever found in the region. The focus remained on religious conversion. The first Spanish missions in Texas were founded in the 1680s around present-day San Angelo, El Paso and Presidio, near the New Mexico settlements. In the early-1680s, however, conflict emerged in New Mexico, as the Pueblo people rebelled against the Spanish occupation. Spanish colonization nevertheless persisted, and in 1690, new missions were built in East Texas by Alonso de León after the Spanish discovered the French had been encroaching into the territory. In Arizona, the first Spanish settlements were founded in 1691 by the Italian Jesuit missionary Father Eusebio