After Wallenstein's murder in 1634, Horn took some areas in Swabia: in the spring of that year, his troops unsuccessfully laid siege to the imperial city of Überlingen, which would have been a rich and valuable prize. In early September 1634 his forces, and those of Bernard of Saxony, were crushed at the battle of Nördlingen by combined Habsburg and Spanish forces. Horn was taken prisoner and held by the Roman Catholic army in Burghausen Castle until 1642. He was exchanged for three imperial generals.