In 1804 Duke Charles Augustus entered into European politics by marrying his son and heir Charles Frederick to Grand Duchess Maria Pavlovna, sister of Emperor Alexander I of Russia. However, at the same time he joined Prussia in the War of the Fourth Coalition against the French Empire, and after the defeat at the Battle of Jena-Auerstedt was forced to accede the Napoleonic Confederation of the Rhine in 1806. In 1809 Saxe-Weimar and Saxe-Eisenach, which had been united only in the person of the duke, were formally merged into the Duchy of Saxe-Weimar-Eisenach.