The territories owned by Frederick William on his accession were divided into three quite separate districts.[3] The old possessions of the house of Hohenzollern in north Europe consisted of the mark of Brandenburg, subdivided for administrative purposes into the old mark, the middle mark and the new mark, which they had ruled as margraves and as electors since the beginning of the fifteenth century. [Sidenote: =The mark of Brandenburg.=] This country, purely German, was like other German states part of the Empire, subject to the legal authority of the Emperor and had its own diet with vague powers of counsel and control over the elector in local affairs. [Sidenote: =The duchy of East Prussia.=] On the east of the Vistula, altogether outside of the limits of the Empire, was the duchy of east Prussia, which had become the hereditary possession of the Hohenzollerns by one of the accidents of the Reformation. The country belonged to the Order of the Teutonic Knights, and was subject to the suzerainty of Poland, but in 1525 the Knights accepted the Lutheran Reformation, dissolved the Order, and formed their territory into a duchy hereditary in the house of the grand master of the time, count Albert of Hohenzollern. At the beginning of the seventeenth century his line became merged in that of the Brandenburg branch of the family, and the elector of Brandenburg became also duke of east Prussia. Here, as in the mark, the existence of a diet in which sat both nobles and burghers formed a constitutional check on the will of the ruler, a check all the more effective because of the reluctance with which the people of east Prussia and their feudal suzerain the king of Poland had acknowledged the rights of the Brandenburg branch to the duchy. But the territorial claims of the young elector did not stop with the German mark of Brandenburg, the Polish duchy of east Prussia, and the succession to the German duchy of Pomerania. [Sidenote: =Duchy of Cleves.=] Within the limits of the Empire, stretching along both banks of the Rhine, in the neighbourhood of Köln, lay the duchies of Cleves, Jülich, Berg and Mark, to which the elector of Brandenburg and the count of Neuburg had put in claims as we have seen in 1609 and thereby very nearly precipitated the great war. By the treaty of Xanten, concluded in 1614 and practically renewed in 1630, the disputed territory was divided between the claimants, and the duchies of Cleves Mark and Ravensberg fell to the share of Brandenburg. During the war, however, Brandenburg was unable to make its power recognised over its new domains. The country was for some time the battle-ground of the Spaniards and the Dutch. As the tide of war rolled away from the lower Rhine it was occupied and practically administered by the Dutch, and when peace was restored Frederick William found himself obliged to assert what was to all intents and purposes a new sovereignty. [Sidenote: =Aims of Frederick William.=]