In March 1634 William Louis was at the meeting in Frankfurt, where Oxenstierna tried to win over the Electors of Saxony and Brandenburg to join the Heilbronn League. Here, the Nassau brothers also reached a compromise with the Lords of Geroldseck about the ownership rights to Lahr. On 7 June William Louis sealed the alliance with France. The negotiations in Frankfurt came to an abrupt end when Emperor Ferdinand II won the Battle of Nördlingen on 6 September 1634. As the imperial army of the Middle Rhine approached, the counts brought their archives to a secure place in Frankfurt and then proceeded to Kirchheim. They had to give up their possessions of the right bank of the Rhine.