When King John III dies in 1592 his son Sigismund accede to the Swedish throne. Conflict arose when Duke Charles, the living son of Gustav Vasa, did not approve of the accession of Sigismund, his nephew, and a Catholic, to the government. Sigismund was related to the Catholic Habsburgs who were attempting to control the Protestant princes of Germany. Religious fervor, economic self-interest, and political and dynastic self-preservation all motivated the Swedes to help the Protestants in the Thirty Years' War.