Ethnic conflicts, economic volatility, and declining empires led to war in the early 20th century. Radical ideologies promoted ethnic cleansing, while Social Darwinism was applied to justify racism between ethnic groups. Industrialization created economic instability and workers' movements. Competing imperialist nations formed militaristic alliances, inflaming nationalism. This volatile environment, combined with racism used to justify genocide and war, caused World War I and II, making the first half of the 20th century the deadliest in human history.