Wikipedia:Selected anniversaries/December 20
- 1803 – As part of the Louisiana Purchase, New Orleans was transferred from France to the United States.
- 1917 – The Cheka, the first Soviet secret police, was founded. Felix Dzerzhinsky was appointed as its leader.
- 1951 – Experimental Breeder Reactor I near Arco, Idaho, USA became the world's first electricity-generating nuclear power plant when it produced sufficient electricity to illuminate four 200-watt light bulbs (pictured).
- 1995 – The NATO-led IFOR began peacekeeping in Bosnia and Herzegovina.
- 1999 – Portugal transferred sovereignty of Macau to the People's Republic of China.
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