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On January 21, 1793, King Louis XVI of France was guillotined in the Place de la Révolution in Paris before a crowd of thousands, becoming the first European monarch to be executed by his own subjects. The regicide marked the point of no return of the French Revolution and shocked European courts, which united against republican France.

On January 21, 1954, the USS Nautilus was launched in Groton, Connecticut, becoming the world's first nuclear-powered submarine. The Nautilus could remain submerged indefinitely and in 1958 made the first underwater transit beneath the ice of the North Pole, covering 3,200 kilometres under the Arctic. Nuclear submarines transformed naval strategy during the Cold War.

On January 21, 1976, Concorde made its first simultaneous commercial flights — Paris-Dakar and London-Bahrain — launching the era of supersonic passenger aviation. The aircraft, capable of crossing the Atlantic in under four hours, was a symbol of luxury and technology, though its high operating costs and noise pollution limited its commercial impact.