An Imperial Diet was arranged for 1641 in Regensburg, where the estates discussed possible peace arrangements. It turned out to be problematic that the emperor had excluded princes, who had previously been on the opposing side, as well as the Protestant administrators of various princes of the Imperial Diet. After all, it finally succeeded in agreeing all imperial estates with the exception of the Electoral Palatinate, the Duchy of Brunswick-Lüneburg and Hesse-Kassel to the resolutions of the Diet. In 1641, a preliminary peace was signed in Hamburg between Ferdinand, Spain, France and Sweden and a final peace congress was to convene in Osnabrück and Münster. An alliance between Sweden and France was fully effective since 1642. The Swedes won the Battle of Breitenfeld in 1642. One year later, Ferdinand was forced to conclude an armistice with Sweden, while France advanced into the Spanish Netherlands.