Each branch of the Armed Forces has historically had different policies regarding racial segregation. Although Executive Order 9981 officially ended segregation in the Armed Forces in 1948, following World War II, some forms of racial segregation continued until after the Korean War.
Example:
With the exception of 18 female African-American nurses who had served in World War I, the Army Nurse Corps, established in 1901, remained white until 1941, when pressure from the National Association of Colored Graduate Nurses, and Eleanor Roosevelt, caused the Army to admit black nurses.
World War II forced Americans to confront the fact that African-Americans were dying for the United States in battle but when they came home, they were subjected to horrific discrimination.
The greatest advantage that the untied had over the central powers that helped the Allies win World War 1 was a great manufacturing potential--another was that they were isolated from invasion.