Hermann joined the German Crusade of 1197 but returned on news of Henry VI's death. In the wars between the rival kings, Philip of Swabia and Otto of Brunswick (1198-1208), Hermann's support was purchased by the emperor's brother, Duke Philip of Swabia, but as soon as Philip's cause appeared to be weakening he transferred his allegiance to Otto of Brunswick, the later Emperor Otto IV. Philip accordingly invaded Thuringia in 1204 and compelled Hermann to come to terms by which he surrendered the lands he had obtained in 1198. After the death of Philip and the recognition of Otto, Hermann was among the princes who assembled at Nuremberg in 1211 and invited Frederick of Hohenstaufen, King of Sicily (afterwards Emperor Frederick II), to come to Germany and assume the crown. In consequence of this step the Saxons attacked Thuringia, but the landgrave was saved by Frederick's arrival in Germany in 1212.