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What happened to Frederick I’s army during his 1166 expedition to Rome?

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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.

During his 1166 expedition to Rome, Frederick I marched with a large force to the city in an attempt to find a decisive solution to his Italian policy. However, his plans were thwarted when a devastating outbreak of malaria struck his troops, which effectively ended the campaign.1