 In 1637 Werth was once more in the Rhine valley, destroying convoys, relieving besieged towns and surprising the enemy's camps.In February 1638 he defeated the Weimar troops in an engagement at Rheinfelden, but shortly afterwards was made prisoner by Bernhard of Saxe-Weimar.His hopes of being exchanged for the Swedish field marshal Gustaf Horn were disappointed for Bernhard had to deliver up his captive to the French.