The need for cheap Mexican labor — particularly in agriculture — was formalized in World War II with the Bracero Treaty. This 1942 agreement formalized a program of temporary Mexican immigration for work in the fields of Texas, California and the Southwest. Working conditions were generally poor and often appalling. Likewise, wages were low for the braceros. Yet the prosperous development of American agriculture in the Southwest was largely done by the hands — and backs — of Mexican workers.

From 1924 until its culmination in 1964, the Bracero program brought four million Mexican workers into the United States. Bracero contracts were controlled by independent farm associations, and abuse and exploitation was rampant. The contracts were in English, and often the workers could not read what they were signing.

In the early 1950s, the rapid rise in illegal immigrants led the government to order the Border Patrol to crack down. Under the supervision of Gen. Joseph Swing, new head of the Immigration and Naturalization Service, "Operation Wetback" was planned and implemented in 1954.

Operation Wetback deported an estimated 3.8 million immigrants, primarily Mexicans. The broad sweep of the operation engulfed not only illegal aliens, but also their U.S.-born children, who were by law U.S. citizens. Tactics included stopping "Mexican-looking" people on the street and asking for identification. These and other heavy-handed procedures led to the termination of Operation Wetback.