Under colonial law, black Angolans were forbidden from forming political parties or labour unions. The first nationalist movements did not take root until after World War II, spearheaded by a largely Westernised and Portuguese-speaking urban class, which included many mestiços. During the early 1960s they were joined by other associations stemming from "ad hoc" labour activism in the rural workforce. Portugal's refusal to address increasing Angolan demands for self-determination provoked an armed conflict, which erupted in 1961 with the Baixa de Cassanje revolt and gradually evolved into a protracted war of independence that persisted for the next twelve years. Throughout the conflict, three militant nationalist movements with their own partisan guerrilla wings emerged from the fighting between the Portuguese government and local forces, supported to varying degrees by the Portuguese Communist Party.