In 1806–1809 Schwarzenberg served as the Austrian ambassador to Russia. He returned to Austria in time to take part in the Battle of Wagram (July 1809), leading a cavalry division in the Reserve Corps and was soon afterwards promoted to general of cavalry. After the signing of Treaty of Schönbrunn (14 October 1809), he was sent to Paris to negotiate the 1810 marriage between Napoleon and the Archduchess Marie Louise of Austria. The prince gave a ball in honour of the bride on 1 July 1810, which ended in a fire that killed many of the guests, including his own sister-in-law, wife of his older brother, Joseph.