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The 20th (twentieth) century began on January 1, 1901 (MCMI), and ended on December 31, 2000 (MM).[1] It was the tenth and final century of the 2nd millennium. The 20th century was dominated by significant events that defined the era: Spanish flu pandemic, World War I and World War II, nuclear weapons, nuclear power and space exploration, nationalism and decolonization, the Cold War and post-Cold War conflicts.