He was an active participator in the East Frankish government of Louis the German (who appointed him), Charles the Fat, and Arnulf of Carinthia.
In 871, Louis the German held an assembly at Frankfurt and from there sent Arn and Ruodolt, Margrave of the Nordgau, to defend the border between the Duchy of Bavaria and Great Moravia because he had heard that the Moravians were planning an invasion.
[3] In 884, Arn and Henry of Franconia led the forces of all East Francia against a Viking army invading Saxony and were victorious.
[4] In 892, Arn, on the advice of Poppo, Duke of Thuringia, had undertaken an expedition against the Wends and was killed, either during a mass on the Chemnitz near Frankenburg or, after withdrawing to Sandberg (perhaps Wiederau or Taurastein), in a decisive battle with the Slavs.
[5] Poppo was deposed from his office for his poor counsel and Arn was replaced by Rudolf, a member of the Conradine family.