The first Polish territory acquired was Pomerania. The career of the knights was easy and successful till the union of Poland and Lithuania in 1386. In 1410, at the battle called by the names both of Grünwald and Tannenberg, the power of the order was broken. Some years later Pomerania was returned to Poland, and the order was allowed to remain in East Prussia in the position of a vassal to the Commonwealth. In this reduced state the knights lived for a time, tried to gain allies, but could not; the most they did--and that was the best for the German cause--was to induce Albert, a member of the Franconian branch of the Hohenzollerns, to become grand master. He began to reorganize the order, and tried to shake off allegiance to Poland; but finding no aid in the Empire or elsewhere, he acted on Luther's advice to introduce Protestantism and convert Prussia into a secular and hereditary duchy. This he did in 1525. Poland, with a simplicity quite equal to that of Konrad, who called in the order at first, permitted the change. The military monks married, and were converted into hereditary nobles. Albert became Duke of Prussia, and took the oath of allegiance to Poland. Later the Hohenzollerns of Brandenburg inherited the duchy, became feudatories of Poland as well as electors at home. This was the position during the war between Sweden and Poland described in THE DELUGE. Frederick William, known as the Great Elector, was ruling at that time in Brandenburg and Prussia. He acted with great adroitness and success; paying no attention to his oath as vassal, he took the part of one side, and then of the other when he saw fit. He fought on the Swedish side in the three days' battle around Warsaw in which Yan Kazimir was defeated. This service was to be rewarded by the independence of Prussia. Hardly had the scale turned in favor of Poland when the Great Elector assisted Yan Kazimir against Sweden; and in the treaty of Wehlau (1657) Poland relinquished its rights over Prussia, which thus became sovereign and independent in Europe. This most important change was confirmed three years later at the peace of Oliva. Frederick, son of the Great Elector, was crowned "King in Prussia" at Königsberg in 1701. The Elector of Brandenburg became king in that territory in which he had no suzerain. At the first division of Poland, Royal Prussia of THE DELUGE, the territory lying between the Vistula and Brandenburg, went to the new kingdom; and Brandenburg, Pomerania, and Prussia became continuous territory. The early success of the Teutonic Knights was so great that in the third half century of their rule on the Baltic their power overshadowed Poland, which was thus seriously threatened. Toward the end of the fourteenth century, however (1386), the Poles escaped imminent danger by their union with Lithuania and Russia. Through this most important connection they rose at once from a position of peril to one of safety and power.