What legal and ethical arguments did white Americans use to justify segregations?

The argument that Thurgood Marshall used to challenge the legality of segregation in Brown v. Board of Education was that “separate but equal” facilities were unequal and that it did great damage to black children as segregation was a form of discrimination that instilled a sense of inferiority to African Americans children and undermined their self-esteem. Consequently, it was a law that violated the 14th Amendment, an amendment that guaranteed all citizens equal protection of the laws.

by 1890, the united states had by far the world's most productive economy. american industry produced twice as much as its closest competitor--britain. but the united states was not a great military or diplomatic power. its army numbered less than 30,000 troops, and its navy had only about 10,000 seamen. britain's army was five times the size of its american counterpart, and its navy was ten times bigger. the united states' military was small because the country was situated between two large oceans and was surrounded by weak or friendly nations. it faced no serious military threats and had little interest in asserting military power overseas.