Aside from Mexicans, there is also people from: Guatemala, El Salvador, Honduras, Nicaragua and Panama crossing US Boarders
In the past year there has been the largest decline of undocumented people entering the US since the 1960's.
info is from,
http://www.pbs.org/kpbs/theborder/history/timeline/17.html
Before Mexican workers supported American agriculture, it was the Chinese who filled the labor hole. Nearly 200,000 Chinese were legally contracted to cultivate California fields, until the Chinese Exclusion Act. Then it was the Japanese who replaced the Chinese as field hands.
Between 1850 and 1880, 55,000 Mexican workers immigrated to the United States to become field hands in regions that had, until very recently, belonged to Mexico. The institution of Mexican workers in the United States was well established at this time in commercial agriculture, the mining industry, light industry and the railroad. The working conditions and salaries of the Mexicans were poor.
The presence of Mexican workers in the American labor scene started with the construction of the railroad between Mexico and the U.S. That presence grew between 1880 and 1890. As much as 60 percent of the railway working crews were Mexican. Rodolfo Tuiran, in his paper "Past and Present of the Mexican Immigration to the United States", reports that the initial flood of migrant workers to the United States were mainly skilled miners, work hands from cattle ranches in Mexico, indentured servants fleeing Mexican farms, small independent producers who were forced north by natural disasters or Indian raids and workers affected by the War of Secession.
In the aftermath of the Mexican Revolution in 1910, the Mexican government was unable to improve the lives of its citizens. By the late 1930s, the crop fields in Mexico were harvesting smaller and smaller bounties, and employment became scarce. The Mexican peasant needed to look elsewhere for survival. World War I also stoked the fire of Mexican immigration, since Mexican workers performed well in the industry and service fields, working in trades such as machinists, mechanics, painters and plumbers. These years were ripe with employment opportunities for Mexicans because much of the U.S. labor force was overseas fighting the war. Agencies in Mexico recruited for the railway and agriculture industries in the United States.
Mexican workers’ complaints about the abuse of their labor rights eventually led the Mexican government to action. Led by Venustiano Carranza in 1920, the Mexican government composed a model contract that guaranteed Mexican workers certain rights named in the Mexican Political Constitution. The contract demanded that U.S. ranchers allow workers to bring their families along during the period of the contract. No worker was allowed to leave for the United States without a contract, signed by an immigration official, which stated the rate of pay, work schedule, place of employment and other similar conditions. Thus, this became the first de facto Bracero Program between the two countries.
In 1924, the U.S. Border Patrol was created, an event which would have a significant impact on the lives of Mexican workers. Though the public did not immediately view Mexicans as "illegal aliens," the law now stated that undocumented workers were fugitives. With the advent of the Border Patrol, the definition "illegal alien" is born, and many Mexican citizens north of the border are subject to much suspicion.
The Mexican work force was critical in developing the economy and prosperity of the United States. The Mexican workers in numerous accounts were regarded as strong and efficient. As well, they were willing to work for low wages, in working conditions that were questionably humane. Another measure of control was imposed on the Mexican immigrant workers during the depression: visas were denied to all Mexicans who failed to prove they had secure employment in the United States. The Mexicans who were deported under this act were warned that if they came back to the United States, they would be considered outlaws.
It seemed whenever the United States found a reason to close the door on Mexican immigration, a historic event would force them to reopen that door. Such was the case when the United States entered World War II. In 1942, the United States was heading to war with the fascist powers of Europe. Labor was siphoned from all areas of United States industry and poured into those which supported the war efforts. Also in that year, the United States signed the Bracero Treaty which reopened the floodgates for legal immigration of Mexican laborers. Between the period of 1942 and 1964, millions of Mexicans were imported into the U.S. as "braceros" under the Bracero Program to work temporarily on contract to United States growers and ranchers.
Under the Bracero Program, more than 4 million Mexican farm workers came to work the fields of the United States. Impoverished Mexicans fled their rural communities and traveled north to work as braceros. It was mainly by the Mexican hand that America became the most lush agricultural center in the world.
The braceros were allowed to return to their native lands only in case of emergency, and required written permission from their employer. When the contracts expired, the braceros were mandated to hand over their permits and return to Mexico. The braceros in the United States were busy thinning sugar beets, picking cucumbers and tomatoes and weeding and picking cotton.
At the end of World War II, Mexican workers were ousted from their jobs by workers coming out of wartime industries and by returning servicemen. By 1947, the Emergency Farm Labor Service was working on decreasing the amount of Mexican labor imported. By the 1960s, an overflow of "illegal" agricultural workers along with the invention of the mechanical cotton harvester, diminished the practicality and appeal of the bracero program. These events, added to the gross humanitarian violations of bracero employers, brought the program to an end in 1964.