Frederick II came to power as the Holy Roman Emperor in 1215.
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The longest reigning Holy Roman Emperor was Frederick III, the
first emperor of the house of Habsburg. The reigned for 69 years
and 70 days: 2 February 1440 -- 19 August 1493.
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It refers to Frederick 1 Barbarossa who was a German Holy Roman
Emperor.
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He was given the title of Holy Roman Emperor.
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The term "Holy" was used by emperor Frederick Barbarossa in the
12th century as a response to the pope who technically called the
emperor one of his vassals. The emperor on the other hand felt
himself as highest sovereign on earth.