August 20 in History

Explanation: Ramón Mercader, a Spanish Soviet agent, attacked Trotsky with an ice axe in Mexico City. Trotsky died the following day from his wounds. Mercader was sentenced to 20 years in prison in Mexico and later received the title of Hero of the Soviet Union.

Explanation: Voyager 2 was launched sixteen days before its sister probe Voyager 1, but on a different trajectory. It remains to this day the only human-made craft to have flown past all four giant planets: Jupiter, Saturn, Uranus, and Neptune. In 2018, it officially left the heliosphere.

Explanation: The Prague Spring was a political reform movement led by Alexander Dubček, General Secretary of the Czechoslovak Communist Party. He wanted to establish 'socialism with a human face'. The Soviet intervention was justified by the Brezhnev Doctrine, which stated that the USSR could intervene in any socialist country threatening the Eastern Bloc.

Explanation: Pierre de Fermat, a magistrate by profession, is one of the founders of number theory. His famous "last theorem", noted in the margin of a copy of Diophantus's Arithmetica, stated that there is no non-zero integer solution to the equation x^n + y^n = z^n for n > 2. This theorem was only proved in 1995 by Andrew Wiles.

Explanation: Darwin presented his theory jointly with Alfred Russel Wallace, who had independently developed similar ideas. This joint presentation at the Linnean Society of London took place on July 1, 1858, but the publication in the journal appeared in August. Darwin then published his major work, "On the Origin of Species", in November 1859.

Explanation: Senegal gained its independence from France on August 20, 1960, after briefly forming the Mali Federation with French Sudan. Léopold Sédar Senghor became its first president. Dakar, the former capital of French West Africa, remained the capital of the new independent state.

Explanation: Howard Phillips Lovecraft, born in Providence (Rhode Island), invented the Cthulhu mythos and the Necronomicon, a fictional grimoire. Little recognized during his lifetime, he deeply influenced the fantastic and horror literature of the 20th century. Authors like Stephen King and Michel Houellebecq have claimed his influence.

Explanation: The NFL was founded in 1920 in Canton, Ohio, under the name American Professional Football Association, before adopting its current name in 1922. It is now the highest-revenue professional sports league in the world, with 32 franchises split between the NFC and the AFC.

Explanation: Rajiv Gandhi was an airline pilot for Air India before entering politics after the assassination of his brother Sanjay in 1980. He became Prime Minister in 1984 following the assassination of his mother Indira Gandhi. He was himself assassinated in 1991 in a suicide attack carried out by a Tamil Tigers militant from Sri Lanka.

Explanation: The Thirty Years' War (1618-1648) was one of the most devastating conflicts in European history, ravaging mainly the Germanic states. The Battle of Lens, won by the Grand Condé against the Spanish, was one of the last major engagements of the conflict. The Peace of Westphalia, signed a few months later, ended this war and redrawn the map of Europe.