Decolonization may involve either nonviolent revolution or national liberation wars by pro-independence groups. It may be or involve the intervention of foreign powers acting individually or through international bodies such as the United Nations. Although examples of decolonization can be found as early as the writings of Thucydides, there have been several particularly active periods of decolonization in modern times. These include the breakup of the Spanish Empire in the 19th century; of the German, Austro-Hungarian, Ottoman, and Russian empires following World War I; of the British, French, Dutch, Portuguese, Belgian, Italian, and Japanese colonial empires following World War II; and of the Soviet Union at the end of the Cold War.