In 1176, at the
Battle of Legnano, Frederick I lost his Italian campaign, blaming it on Henry
the Lion, who had refused to support him. Henry was busy extending and defending his own reign against the quarrelling clan of the Ascanians. Wichmann visited Frederick I in Italy in order to fathom in how far Siegfried's claim to the Bremian See could be enforced – e.g. in the scope of the
Peace of Venice. Frederick I declared Baldwin's investiture as Archbishop and all his alienations of archiepiscopal possessions to be invalid. When in 1178 Baldwin received the official notification of his dismissal, he died. In 1179 Siegfried attended the
Third Council of the Lateran in
Rome, while the Bremian Chapter elected another Guelphic partisan,
Berthold, for Archbishop. Both Frederick I and Alexander III originally wanted to confirm this knowledgeable man in his new position. But when in 1179 Berthold arrived in Rome to gain his papal confirmation, Alexander III declared Berthold's election null and void. Inferior in warfare, many of Henry's enemies litigated him in lawsuits. Henry absented at the trials, to which he had been summoned. Thus the
Diet decided to bring Henry before a judge, using military violence ( 1180-1181). Frederick I
Barbarossa and his allies, many of them vassals and former supporters of Duke Henry
the Lion, had defeated him. In 1180 Frederick I
Barbarossa stripped Henry
the Lion of his duchies. In 1182, he and his wife went into exile. In 1180, at the Diet in
Gelnhausen, the attending princes and
Frederick I Barbarossa decided to partition Saxony in some dozens of territories of
imperial immediacy, allotting each territory to that one of his allies, who had conquered them before from
Henry the Lion and his remaining supporters. Otto I wielded his influence at the Diet. His and Siegfried's brother Bernhard, one of the most steadfast warriors against Henry
the Lion, was provided with the later on so-called
younger Duchy of Saxony (1180 - 1296), thus becoming
Bernhard III, Duke of Saxony. In 1260, with effect from 1296 on, its later rulers split the
younger Duchy into the Duchies of
Saxe-Wittenberg () and
Saxe-Lauenburg (), the latter belonging in religious respect to the
archdiocese of Bremen. The Gelnhausen Diet (1180) confirmed Siegfried as Archbishop upgraded part of the diocesan territory to form the
Prince-Archbishopric of Bremen of
imperial immediacy. Thus the
Prince-Archbishopric of Bremen became one of the successor states of the
old Duchy of Saxony, holding only a small part of its former territory. Since the deposed Henry
the Lion had entrenched in his last Saxon stronghold, the city of
Stade, Otto I and Bernhard III militarily supported their brother Siegfried to de facto gain the power in all the Prince-Archbishopric. ==Siegfried as Prince-Archbishop of Bremen==