Despite the Brown vs. Board of Education case ruling that segregated schools were illegal, many Southern states tried to prevent the desegregation of schools.
For example, in Arkansas the governor Orval Faubus used the Arkansas State Guard (aka military) to prevent nine African-American students from entering Central High School. Central High School was supposed to be one of the first schools to become integrated until the governor used his power to stop it from happening.
Ultimately, President Dwight D. Eisenhower would get involved and he would use the US National Guard to escort these students into school.