Profiting by these reverses the elector then undertook a series of internal reforms, tending to strengthen the central authority, and to mitigate the constant lack of money, which was perhaps his chief obstacle to success; a work in which he was aided by George, count of Waldeck (1620-1692), who became his chief adviser about this time. In 1651 the powers of the state council were extended to include all the lands under the elector's rule; and a special committee was appointed to effect financial economies, and so to augment the electoral resources. In imperial politics Frederick William supported the election of Ferdinand, son of the emperor Ferdinand III., as king of the Romans in 1653; but when the emperor failed to fulfil his promises, influenced by Waldeck, he acted in opposition to the imperial interests, and even formed a plan for a great alliance against the Habsburgs. These projects were disturbed by the war which broke out in 1655 between Sweden and Poland. In this struggle the elector fought first on one side and then on the other; but the important consequences of his conduct belong rather to the history of the duchy of Prussia (q.v.). The transfer of the elector's support from Sweden to Poland in 1656 was followed by the fall from power of Waldeck, who was succeeded by Otto von Schwerin (1616-1679), under whose influence the elector's relations with the emperor became more cordial.
The increase in the prestige of Brandenburg was due chiefly to his army, which was gradually brought to a high state of efficiency. A proper organization was established to superintend the pay and maintenance of the soldiers, and they were commanded by experienced officers, among others by Georg Derfflingen (1606-1695), and Otto von Sparr (1605-1668). The general poverty, however, made the estates reluctant to support a standing army, and after the peace of Oliva in 1660, it was reduced to about 3500 men. The continual difficulties with the estates of his different dominions had harassed and hampered the elector, and the general peace which followed the treaty of Oliva offered a favourable opportunity to curtail their powers. Undaunted by two previous rebuffs he attacked the estates of Cleves, and by a display of force gained a substantial victory. Some important privileges were annulled, and he obtained a considerable sum of money. The _Landtag_ of Brandenburg was not cowed so easily into submission, but an increase of revenue was obtained, and the stubborn struggle which ensued in Prussia ended in a victory for the ruler. This increased income enabled the elector to take a more considerable part in European politics. In 1663 he assisted the imperialists in their struggle with the Turks; in 1666 the dispute over Cleves, Mark and Ravensberg was finally settled, and Brandenburg were confirmed in the possession of these lands; and in the same year a reconciliation was effected with Sweden. Several disputes which threatened to disturb the peace of the Empire were settled through his mediation, and he compelled the citizens of Magdeburg to do homage to him. In religious matters he interceded with the emperor and the diet for the Protestants, and sought, but without success, to bring about a reconciliation between Lutherans and Calvinists in Brandenburg.