Final answer:
The practice of placing men into military units based on the color of their skin is known as ethnic segregation. This occurred before the desegregation of the military in 1948, with examples including the all-Black units in World War I.
Step-by-step explanation:
The term that describes the practice of placing men into military units based on the color of their skin is ethnic segregation. This practice has been part of a wider culture of prejudice, constituting discrimination within the ranks of the military. Throughout history, especially during times such as World War I and before the desegregation of the military in 1948, African American troops and other minority soldiers were often segregated based on race and ethnicity.
Racial drafting was a term used to describe the enlistment and organization of troops along racial lines, leading to all-Black units or units composed of other minorities like Mexican Americans or Filipinos. Colorism, which is an expression of prejudice wherein certain skin tones within a racial group are preferred over others, also played a role in discrimination but is not the specific practice of segregating military units.