During the Thirty Years' War, he started as supporter of the Catholic League but his growing tense with the Spanish Habsburgs led him on the side of France and in opposition to Emperor Ferdinand II.[2] Spanish Habsburg troops under Christopher of East Frisia retook Trier by surprise in 1635, and Sötern was subsequently imprisoned in Linz from 1635 to 1645.He ordered Turenne to capture the still Spanish occupied Trier and left Philippsburg to French troops against his agreement with the Emperor.[2] After French troops took Trier in November 1645 and once more left a garrison, most of the cathedral chapter feared for their safety and fled to Cologne.The exiled cathedral chapter protested and organised troops to take control of the Electorate and force Sötern to accept an election of the coadjutor.