With as much as immigration and "illegal aliens" are a hot political topic these days, in the very early 1900s, Mexican workers were eagerly sought to work the US farms and fields and factories. Between 1910 and 1930, the "official" number of Mexican immigrants jumped from 200,000 to 600,000 (according to the US Census Bureau). The real numbers were probably a lot higher because of the long and relatively unsecured border between the two countries. Mexican nationals were eager to escape the relative chaos of the Mexican Revolution. American employers loved the Mexican workers because they worked cheap, didn't complain about working conditions, and were excellent workers.
American employers got too complacent about the immigrants' working conditions. Mexico finally had to insist that the employers give each worker a contract "which specified the rate of pay, work schedule, place of employment and other related conditions." The same contract guaranteed the worker the right to take his family along with him, for the term of the contract. (Esperanza's mother had her contract, and she discussed the "accompanying family" issue with Esperanza shortly after they arrived in the United States.)
When the Depression hit, and the drought, and then the Dust Bowl, things changed. There were a lot of very hungry refugees from the midwest and east, eager to work for a pittance.
PBS has a great timeline showing the different attitudes toward the US/Mexican border over the years. Scroll right to move later in history!