A Roman garrison in the 1st cent. B.C., Cologne was made a Roman colony in A.D. 50 by Emperor Claudius, who named it Colonia Claudia Ara Agrippinensis for his wife, Agrippina. The city passed under Frankish control in the 5th cent. The episcopal see, established there in the 4th cent., was made an archdiocese under Charlemagne. Its archbishops, who later ruled a strip of land on the west bank of the Rhine as princes of the Holy Roman Empire, acquired great power and ranked third among the electors. The archbishops' constant feuds with the lay citizenry resulted in the transfer (mid-13th cent.) of their residence to nearby Brühl, then to Bonn.
Cologne was self-governing after 1288, became a free imperial city in 1475, and, as a member of the Hanseatic League, flourished as a commercial center until the 16th cent. Its decline was hastened by the expulsion of the Jews (15th cent.) and the restrictions imposed on Protestants (16th cent.). Cologne was seized by the French in 1794, and the archbishopric was officially secularized in 1801. The city passed to Prussia in 1815, and in 1821 the archdiocese was reorganized.
In the 19th cent. Cologne prospered again as an industrial center and as the main transit port and depot of NW Germany. The industrial town of Deutz (noted for the manufacture of motors), on the east bank of the Rhine, was united with Old Cologne, on the west bank. Old Cologne, with its numerous historic buildings, was severely damaged by aerial bombardment in World War II.