birth control techniques, women became more independent throughout the century. Industrial warfare greatly increased in its scale and complexity during the first half of the 20th century. Notable developments included chemical warfare, the introduction of military aviation and the widespread use of submarines. The introduction of nuclear warfare in the mid-20th century marked the definite transition to modern warfare. Civil wars occurred in many nations. A violent civil war broke out in Spain in 1936 when General Francisco Franco rebelled against the Second Spanish Republic. Many consider this war as a testing battleground for World War II, as the fascist armies bombed some Spanish territories. The Great Depression in the 1930s led to the rise of Fascism and Nazism in Europe. World War II (1939–1945) involved Eastern Asia and the Pacific, in the form of Japanese aggression against China and the United States. Civilians also suffered greatly in World War II, due to the aerial bombing of cities on both sides, and the German genocide of the Jews and others, known as the Holocaust. During World War I, in the Russian Revolution of 1917, 300 years of Romanov reign were ended and the Bolsheviks, under the leadership of Vladimir Lenin, established the world's first Communist state. After the Soviet Union's involvement in World War II, communism became a major force in global politics, notably in Eastern Europe, China, Indochina and Cuba, where communist parties gained near-absolute power. The Cold War (1947–1989) involved an arms race and increasing competition between the two major players in the world: the Soviet Union and the United States. This competition included the development and improvement of nuclear weapons and the Space Race. This led to the proxy wars with the Western bloc, including wars in Korea (1950–1953) and Vietnam (1957–1975). The Soviet authorities caused the deaths of millions of their own citizens to eliminate domestic opposition. More than 18 million people passed through the Gulag, with a further 6 million being exiled to remote areas of the Soviet Union. The civil rights movement in the United States and the movement against apartheid in South Africa challenged racial segregation in those countries. The two world wars led to efforts to increase international cooperation, notably through the founding of the League of Nations after World War I, and its successor, the United Nations, after World War II. Nationalist movements in the subcontinent led to the independence and