<Ul> <Li> 19th century </Li> <Li> 20th century </Li> <Li> 21st century </Li> </Ul> <Tr> <Th> State leaders: </Th> <Td> <Ul> <Li> 19th century </Li> <Li> 20th century </Li> <Li> 21st century </Li> </Ul> </Td> </Tr> <Ul> <Li> 19th century </Li> <Li> 20th century </Li> <Li> 21st century </Li> </Ul> <Tr> <Th> Decades: </Th> <Td> <Ul> <Li> 1900s </Li> <Li> 1910s </Li> <Li> 1920s </Li> <Li> 1930s </Li> <Li> 1940s </Li> </Ul> <Ul> <Li> 1950s </Li> <Li> 1960s </Li> <Li> 1970s </Li> <Li> 1980s </Li> <Li> 1990s </Li> </Ul> </Td> </Tr>

Which of the following was not a feature of twentieth-century world history