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On October 21, 1805, the British fleet of Admiral Horatio Nelson defeated the Franco-Spanish armada at the Battle of Trafalgar, off the cape of the same name on the Cádiz coast. Nelson was killed during the battle, but his victory secured British naval dominance for more than a century and definitively thwarted Napoleon's invasion plans.

On October 21, 1879, Thomas Edison kept a carbon filament light bulb burning for 13.5 hours in his Menlo Park laboratory, demonstrating the practical viability of electric lighting. Although others had experimented before, Edison was the one who made the light bulb commercially viable and built the electrical distribution system to accompany it.

On October 21, 1797, the frigate USS Constitution was launched in the port of Boston, becoming the oldest warship still afloat in the world. Nicknamed "Old Ironsides" for her resilience in battle, she served in the War of 1812 and was never sunk in combat. She remains today an active commissioned ship of the United States Navy.