Final answer:
America filled its labor needs during labor shortages, especially during wartime, through the Bracero Program, which brought Mexican laborers, and by women and African Americans taking jobs in cities and industrials sectors left vacant by enlisted men.
Step-by-step explanation:
The two sources that provided the manpower to fill America's labor needs during times when there was an increase in demand for workers included the Bracero Program, which brought in Mexican laborers for agriculture and railroad construction, and the migration of Hispanic Americans, women, and African Americans from the South into cities and industries.
Military enlistment during World War II created a labor shortage that was initially addressed by the Bracero Program in 1942. This program allowed Mexican agricultural workers to work in the U.S. under the condition that they would not be drafted into military service. Concurrently, as men went off to fight in the war, there was a need to fill the jobs they left behind. Women entered the workforce in large numbers to take on roles that were previously dominated by men, particularly in manufacturing vital to the war effort.
Additionally, the Manufacturing Belt saw a shift from European immigrant workers to African Americans migrating from the South seeking opportunities in industrial work during and after World War I, when immigration policies became restrictive.