After a power struggle, Bolesław III Wrymouth (r. 1102–1138) became the duke of Poland by defeating his half-brother Zbigniew in 1106–1107. Zbigniew had to leave the country, but received support from Holy Roman Emperor Henry V, who attacked Bolesław's Poland in 1109. Bolesław was able to defend his realm due to his military abilities, determination and alliances, and also because of a societal mobilisation across the social spectrum (see Battle of Głogów). Zbigniew, who later returned, died in mysterious circumstances, perhaps in the summer of 1113. Bolesław's other major achievement was the conquest of all of Mieszko I's Pomerania (of which the remaining eastern part had been lost by Poland from after the death of Mieszko II), a task begun by his father Władysław I Herman and completed by Bolesław around 1123. Szczecin was subdued in a bloody takeover and Western Pomerania up to Rügen, except for the directly incorporated southern part, became Bolesław's fief, to be ruled locally by Wartislaw I, the first duke of the Griffin dynasty.