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.

The elector's relations with Louis XIV. of France are full of interest.
After the conclusion of the war of devolution in 1667, he allied himself
with Louis, and together they agreed to support the candidature of
Wolfgang of Neuburg for the vacant Polish throne. In 1668, moreover, he
refused to join the triple alliance against France, but soon afterwards
became aware of the danger to his country from the aggressive policy of
Louis. The United Provinces were bound to him by religious interests,
political considerations, and family ties alike, and he could not be
indifferent when their position was threatened by France. In spite of
tempting offers from Louis, he was the first to join the Dutch when they
were attacked by Louis in 1672, and conducted an ineffectual campaign on
the Rhine until June 1673, when he was forced to make peace. In July
1674, however, he joined the Empire, the United Provinces and Spain, and
in return for a subsidy, fought against France in Alsace. Meanwhile
Louis had instigated the Swedes to invade Brandenburg, which had been
left to the care of John George II., prince of Anhalt-Dessau. Hastening
from Franconia to defend the electorate, Frederick William gained a
complete victory over a superior number of the enemy at Fehrbellin on
the 28th of June 1675, a great and glorious day for the arms of
Brandenburg. Aided by the imperialists and the Danes, he followed up
this success, and cleared Brandenburg and Pomerania of the Swedes,
capturing Stettin in 1677 and Stralsund in 1678, while an attack made by
Sweden on Prussia was successfully repelled. The general peace of
Nijmwegen was followed by the treaty of St Germain-en-Laye