King of Prussia (1797–1840). After his defeat at the Battle of Jena he was forced by the Treaty of Tilsit (1807) to surrender half his dominions by the creation of the kingdom of Westphalia and the grand duchy of Warsaw. In 1811 he joined Napoleon in the war against Russia but, following the retreat of Napoleon from Moscow, he signed a military alliance with Russia and Austria. From 1807 onwards he supported the efforts for reform made by Stein and Hardenberg, and at the Congress of Vienna he won back for Prussia Westphalia and much of the Rhineland and of Saxony. He signed the Holy Alliance, and became progressively more reactionary during the last years of his reign.