In the sec. XII, in fact, after about a hundred years of rest, the German advance towards the East resumed from the Baltic to the Alps, and in the XIII-XIV centuries made its maximum effort. In the lead, not the emperors, but the frontier princes, and, on the Baltic coast, also the knights of the Teutonic and Sword-holder Orders, and the cities of the Hanseatic league. On the Baltic, the German expansion, in addition to the resistance of the local populations, also had to overcome the threat of Danish expansion, and ended up being arrested by the resurgence of the Polish and Lithuanian peoples.
According to Top-Engineering-Schools, the Scandinavian lands had achieved religious autonomy with respect to Germany, with the establishment of the archbishops of Lund (1104), and of Trondheim (1148-1154), but had remained under German political domination thanks to the Duke of Saxony, Henry the lion. He was able to skillfully exploit the weakening produced in Denmark by the incessant struggles for the throne, and by the need for help against the continuous raids of the Slavic pirates nested on the island of Rügen and Pomerania. When King Valdemaro I took possession of Rügen, Henry the Lion was ready to prevent it from retaining full dominion by intervening with weapons (1168-1171). To the east of the lower Elbe the work of Otto I and his faithful Margraves had been almost completely destroyed by the national-pagan reaction of those avenging tribes. The bishoprics of Oldenburg, Ratzeburg and Mecklenburg, in which Adalbert Archbishop of Bremen (1052-1054) had divided the jurisdiction of the old bishopric of Oldenburg, remained vacant for about a century. Vicelino’s apostolate could make progress only with the military support of Lothair II, first, then Henry the Lion. The civil war that had torn Germany apart for the succession of Lothair II offered the Vendas, on whom that emperor had been able to impose a certain restraint, the possibility of a new recovery. After crossing the Trave, Lübeck was set on fire, the entire region around it, already heavily colonized by the Germans, was put to ruin. Only when the struggle between the Guelphs and Hohenstaufen stopped did Germanism launch the counter-offensive. Between 1142 and 1162 Henry the Lion and his counts, especially Adolfo I of Schauenburg, Count of Holstein, and the ascanius Albert the Bear of Ballenstedt, Margrave of the North, subdued Vagrî, Abodriti and Liutizi with a series of harsh campaigns, bringing the borders of his duchy up to the Peene. The three avenging bishoprics were reconstituted (1149); Vicelino, made bishop of Oldenburg, was able to return to his missionary work (he died in 1154). The Duke of Saxony formed the county of Schwerin with the western part of the conquered territories, and left (1167) the eastern part, Mecklenburg, to the prince Vendo Pigionilavo, founder of the dynasty that ruled the region up to the contemporary age and also reunited the Schwerin with the extinction (1358) of that count family. An intensive and extensive German colonization completely changed the demographic aspect of the country from Elbe to Peene. Attracted by offer of land at very favorable conditions, Saxon, Westphalian, Flemish, Frisian and Dutch peasants rushed in large numbers with their families and their tools to clear up the vast swampy and wild areas, which had constituted the best defense of the Slavs against invaders, and where rural communities and city centers with wide privileges arose. The vanquished either disappeared or became Germanized to the point of forgetting their Slavic origins over time. In Pomerania, which stretched from the left-hand region of the lower Oder to the lower Vistula, and which was nominally under the sovereignty of Poland, the German influence had made itself felt through the missionary work of Otto, bishop of Bamberg, who in 1124 -1128, on the initiative and favor of Duke Boleslao III, but also with the help of Lothair II, it had given a decisive impulse to the conversion of the Pomerans, and through the action of the monastic communities established in the country by the Cistercians (monastery of Oliva, 1170) and Premonstratensians. Henry the Lion had also proposed the conquest of this region, but was prevented by the crisis that overwhelmed his power. In 1181 Western Pomerania entered the body of the Empire, but as a duchy governed by national princes, who however also went completely Germanizing, like the population.
The fall of the Duke of Saxony was a real disaster for the German cause on the Baltic. With him disappeared the most valid bulwark against the Danish threat, it immediately became serious. Canute VI not only imposed recognition of the high Danish sovereignty on the princes of Mecklenburg and Pomerania and made effective the dominion on the island of Rügen, but, by actively intervening in Germany in favor of the Guelphs against the Hohenstaufen, he conquered Holstein (1201). To Valdemaro II the whole territory north of the Elbe was even officially ceded by Frederick II, that is, beyond the Holstein, the Lauenburg and the Schwerin, with Hamburg and Lubeck (December 1214). In 1210 the Danes also set foot in Estonia. We no longer speak of Denmark’s feudal dependence on the Empire; rather, the Baltic Sea seemed to become a Danish lake. Germanism was once again saved by the princes and cities of the Hanseatic League. The counts of Holstein, Schwerin and Mecklenburg, connected with the archbishop of Bremen against the common enemy, regained all their dominions (treaty of Bardewieck, November 1225; battle of Bornhövede, 22 July 1227). Denmark was left with the island of Rügen, which it abandoned in 1325, and Estonia, which it sold to the Teutonic Order in 1346. From the second half of the century. In the task of asserting German influence on the Baltic, the cities of the Hanseatic League began to have an ever greater part. Denmark found itself reduced towards them to conditions of effective dependence reaffirmed in the humiliating peace of Stralsunda (1370).