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For questions spanning a significant proportion of the 20th century. Questions focused on the latter decades should consider using the contemporary-history tag. Individual tags exist for specific decades.

The 20th century began on January 1, 1901, and ended on December 31, 2000. Notable events during this century includes:

  • The British, Chinese, Russian, German, Ottoman, and Austro-Hungarian empires dissolved in the first half of the century, with all but the British, French, and Japanese empires collapsing during the course of World War I.
  • Russia transformed into the communist state of Soviet Union.
  • The inter-war years saw a Great Depression cause a massive disruption to the world economy.
  • World War II broke out, pitting the Allied powers (chiefly the Soviet Union, the United States, and the United Kingdom) against the Axis powers (Nazi Germany, the Empire of Japan, and Italy). This eventually resulted in a total victory for the Allies, at the cost of over 60 million lives and the complete devastation of many nations.
  • Remaining colonial empires dissolved shortly after the war.
  • As a means of preventing future world wars, the United Nations was formed.
  • Competition between the two new superpowers, the Soviet Union and the United States, resulted in the Cold War, which would dominate geopolitical life for the next 45 years.
  • The Soviet Union collapsed internally in 1991, resulting in the United States taking on sole superpower status.