This migration phenomenon is not new, but it has been a defining feature in the relationship of both countries for most of the twentieth century.[36] During World Wars I and II, the United States government approved the recruitment of Mexican workers in their territory, and tolerated unauthorized migration in order to obtain additional farm- and industrial-workers to fill the necessary spots vacated by the population in war, and to supply the increase in the demand for labor. Nonetheless, the United States unilaterally ended the wartime programs - in part as a result of arguments from labor and from civil-rights groups.[37]