 After the Treaty of Tilsit, which Frederick William III of Prussia and Tzar Alexander I of Russia concluded with Napoleon in July 1807, Frederick Augustus was also named duke of Warsaw.Although he had rejected the offer of the kingdom of Poland in 1795 by the Sejm, he could not refuse a Polish title a second time.The Constitution of the Duchy of Warsaw, which Napoleon dictated to Saxony, joined the Duchy of Warsaw hereditarily to the Royal House of Saxony in Article V, which was linked to the Polish Constitution of 1791.