Meanwhile, many Southern politicians expressed outrage at the Court's decisions and promised to resist any federal attempt to force desegregation, a strategy known as massive resistance. Although "Brown" did not mandate immediate school desegregation or bar other "separate but equal" institutions, most observers recognized that the decision marked the beginning of the end for the Jim Crow system. Throughout his years as chief justice, Warren succeeded in keeping decisions concerning segregation unanimous. "Brown" applied only to schools, but soon, the Court enlarged the concept to other state actions by striking down racial classification in many areas. Warren compromised by agreeing to Frankfurter's demand for the Court to go slowly in implementing desegregation. Warren used Frankfurter's suggestion for a 1955 decision ("Brown II") to include the phrase "all deliberate speed." In 1956, after the Montgomery bus boycott, the Supreme Court affirmed a lower court's decision that segregated buses are unconstitutional. Two years later, Warren assigned Brennan to write the Court's opinion in "Cooper v. Aaron". Brennan held that state officials were legally bound to enforce the Court's desegregation ruling in "Brown".