World War II (1939–1945) involved sustained strategic bombing of railways, harbours, cities, workers' and civilian housing, and industrial districts in enemy territory. Strategic bombing as a military strategy is distinct both from close air support of ground forces and from tactical air power. During World War II, many military strategists of air power believed that air forces could win major victories by attacking industrial and political infrastructure, rather than purely military targets. Strategic bombing often involved bombing areas inhabited by civilians, and some campaigns were deliberately designed to target civilian populations in order to terrorize them or to weaken their morale. International law at the outset of World War II did not specifically forbid the aerial bombardment of cities – despite the prior occurrence of such bombing during World War I (1914–1918), the Spanish Civil War (1936–1939), and the Second Sino-Japanese War (1937–1945).