The Franks, who had thrice previously sacked the city, gained permanent possession of it about 455. The city passed to Lor raine in 843, and to the East Frankish kingdom in 870. Hetti, who occupied the see from 814 to 847, is said to have been the first Archbishop of Trier, and Radbod acquired the temporal rights of the counts of Trier in 898. In the loth century Arch bishop Dietrich I. obtained the primacy over Gaul and Germany.
The temporal power of the archbishops was not gained without opposition. The German kings Otto IV. and Conrad IV. granted charters to the city, which, however, admitted the jurisdiction of its archbishop, Baldwin of Luxembourg, in 1308. This prince, a brother of the emperor Henry VII., ruled from 1307 to 1354 and was the real founder of the power of Trier. He raised it to great prosperity by his foresight, and chiefly as a result of the active support he rendered to the emperors Henry VII., Louis the Bavarian, and Charles IV., enlarged his dominions almost to their ultimate extent. He assumed the title of arch-chancellor of Gaul and Arles (or Burgundy), and thenceforward the elector of Trier held the third place in the electoral college. After Baldwin's death the prosperity of Trier was checked by wars and disputes between rival claimants to the see, and in 1456 the estates united for the purpose of restoring order and secured the right of electing their archbishops.
Throughout the middle ages the sancta civitas Trevirorum was a great seat of monastic learning. The university, founded in 1473, existed until 1797. The elector Richard von Greiffenklau (1467-1531) successfully opposed the Reformation and inaugu rated the exhibitions of the holy coat, which called forth the denunciations of Luther but long after his time continued to bring wealth and celebrity to the city. In the latter half of the
16th century education fell into the hands of the Jesuits.
The last elector and archbishop, Clement Wenceslaus (1768 1802), granted toleration to the Protestants in 1782, established his residence at Coblenz in 1786, and fled from the French in In 1814 nearly the whole of the former electoral dominions were given to Prussia. A bishopric was again founded in 1821, but it was placed under Cologne. The area of the former electoral principality covered a broad strip of territory along the lower Saar and the Moselle from its confluence with that river to the Rhine, with a district on the right bank of the Rhine behind Ehrenbreitstein. The chief towns in addition to Trier were Coblenz, Cochem, Beilstein, Oberwesel, Lahnstein, and Sayn. The territory under the spiritual authority of the archbishop included the bishoprics of Metz, Toul, and Verdun, and after 1777 also those of Nancy and St. Die.
At the close of 1918, after the Armistice, the town of Trier was occupied by units of the Third American Army, the last of which returned to the United States in 1923.