After restoring a degree of peace and prosperity in his principality, Galen had to contend with a formidable insurrection on the part of the citizens of Münster; but in 1661 this was solved by the occupation of the city. At the head of the largest ecclesiastical principality of the Holy Roman Empire, the prince-bishop, who maintained a strong army, became an important personage in Europe. In 1664, he was chosen one of the directors of the imperial army raised to fight the Turks, but his troops came too late to fight; after the peace which followed the Christian victory at the Battle of St. Gotthard in August 1664, he aided Charles II of England in his Second Anglo-Dutch War with the Dutch, until the intervention of Louis XIV and Frederick William, Elector of Brandenburg compelled him to make a disadvantageous peace in 1666 in Cleve.