The 20th (twentieth) century began on January 1, 1901 (MCMI), and ended on December 31, 2000 (MM). 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.