When Henry V. died in 1125, Lothair, after a protracted election, was chosen German king at Mainz on the 30th of August 1125. His election was largely owing to the efforts of Adalbert, archbishop of Mainz, and the papal party, who disliked the candidature of Henry's nephew and heir, Frederick II. of Hohenstaufen, duke of Swabia. The new king was crowned at Aix-la-Chapelle on the 13th of September 1125. Before suffering a severe reverse, brought about by his interference in the internal affairs of Bohemia, Lothair requested Frederick of Hohenstaufen to restore to the crown the estates bequeathed to him by the emperor Henry V. Frederick refused, and was placed under the ban. Lothair, unable to capture Nüremberg, gained the support of Henry the Proud, the new duke of Bavaria, by giving him his daughter, Gertrude, in marriage, and that of Conrad, count of Zähringen, by granting him the administration of the kingdom of Burgundy, or Arles. As a counterstroke, however, Conrad of Hohenstaufen, the brother of Frederick, was chosen German king in December 1127, and was quickly recognized in northern Italy. But Lothair gained the upper hand in Germany, and by the end of 1129 the Hohenstaufen strongholds, Nüremberg and Spires, were in his possession. This struggle was accompanied by disturbances in Lorraine, Saxony and Thuringia, but order was soon restored after the resistance of the Hohenstaufen had been beaten down. In 1131 the king led an expedition into Denmark, where one of his vassals had been murdered by Magnus, son of the Danish king, Niels, and where general confusion reigned; but no resistance was offered, and Niels promised to pay tribute to Lothair.