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Why did Frederick I’s campaign against Rome fail after a malaria outbreak?

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Medieval manuscript miniature showing Frederick I as a crowned crusader holding an orb and scepter, with Henry of Schaftlarn kneeling and presenting a book to him. — Frederick I Frederick I as a Crusader with (right) Henry of Schäftlarn dedicating to him a copy of the *History of the First Crusade* by Robert of St. Remy; miniature from a manuscript in the Vatican Library.

In 1166, Frederick I (Barbarossa) led a large military force to Rome in an attempt to secure a decisive resolution to his ongoing conflicts in Italy. However, the campaign failed because a devastating outbreak of malaria struck his troops while they were in the city, effectively ending the emperor’s plans and forcing a retreat.1