A serious Swedish defeat at Nördlingen in September 1634 threatened their participation, leading France to intervene directly. Under the April 1635 Treaty of Compiègne negotiated with Axel Oxenstierna, Richelieu agreed new subsidies for the Swedes. He also hired mercenaries led by Bernhard of Saxe-Weimar for an offensive in the Rhineland and declared war on Spain in May, beginning the 1635 to 1659 Franco-Spanish War.