Marcomanni a Germanic tribe of the Suevic branch, dwelt from the Helvetian border to the Main, and from the Rhine to the Danube. They are first mentioned by Julius Caesar in his Gallic wars (1:51), who reckons them among the forces of Ariovistus, king of the Suevi. The conquests of the Romans brought them into dangerous proximity to the Marcomanni, and induced the latter to seek a new home in modern Bohemia. They were led by Marobodhus, a man of noble rank among them, trained in the Roman armies, and he became their king after the conquest of Bohemia. The Marcomanni quickly acquired influence, and were greatly strengthened by alliances with all the neighboring tribes, so that their power became threatening to the empire. Tiberius concluded a treaty of peace with them, which secured the empire against an attack, but turned against them the hatred of the remaining Germanic tribes. Led by Arminius, these enemies defeated the Marcomanni in A.D. 17, after which date their history presents an almost uninterrupted succession of conflicts. They defeated the emperor Domitian (Dio Cassius, 57, 7), and in A.D. 164 advanced to Aquileia, in Italy. The fruits of a decisive victory over them, won by the generals of M. Aurelius, were lost by a treaty which the emperor Commodus concluded with them (A.D. 180), and they continued to make frequent irruptions into the neighboring provinces of the empire, penetrating in A.D. 270 even to Milan, besieging Ancona, and threatening Rome itself. Their name gradually disappears from history during the 5th century, when the migration of more distant barbarians brought a succession of new peoples into their land.
It is not definitely known how or when they became acquainted with Christianity. Their frequent incursions into the empire doubtless brought them into contact with its disciples, some of whom must have been among their prisoners of war. A statement in the life of St. Ambrose, by Paulinus — which, however, is not confirmed by any contemporaneous author — relates that in the time of that bishop an Italian Christian had visited the Marcomanni, and had awakened the interest of their queen in Christianity to an extent that led her to apply to Ambrose for instruction. He sent, in compliance with her request, a work in the form of a catechism, by which both she and the king were led to embrace Christianity towards the close of the 4th century. See Schrickh, Kirchengesch. 7:347; Hefele, Gesch. d. Einfuhrung des Christenthuums im süd-westl. Deutschland, vol. 7; Tacitus, Annals; Dio Cassius, Hist. Romans 1, 54, and Greek and Roman historians of this period. See also Herzog, Real-Encyklopädie, 9:112; Wetzer u. Welte, Kirchen-Lex. s.v. (G. M.)