The answer is: he defied the federal government and blocked black teenagers from attending Little Rock Central High School
This happened After The Supreme Court of United States issued its historic Brown v. Board of Education decision. The decision make racial segregation based on race become unconstitutional.
Orval Faubus on the other hand , refuse to obey the mandate given by the supreme court. He ordered Several military personnel to surround Little Rock Central High School to prevent black teenagers from attending.