Arnim left the imperial service on account of the Edict of Restitution and Wallenstein's dismissal. He entered that of the Elector John George of Saxony, and was in command of the left wing of the army of Gustavus Adolphus at Breitenfeld (1631); indeed the alliance of Sweden and Saxony, two Protestant powers, in the cause of their common religion was largely his work. He then invaded Bohemia, took Prague, and was victorious at Nimburg (today Nymburk), and in 1632 returned to Saxony, then fought in Brandenburg and Silesia. He was one of the principal agents in the negotiations between John George and Wallenstein, which were terminated by the latter's death in 1634. After this, he defeated the imperial army at Liegnitz and operated in conjunction with Bauer in Bohemia.