William occupied Hersfeld and the Mainz estate of Fritzlar; however, when he and Gustavus Adolphus entered the city of Frankfurt, his cousin Landgrave George II of Hesse-Darmstadt, allied with Emperor Ferdinand II, was alarmed and started negotiations with the Swedish king. William's expectations proved to be false, when Gustavus Adolphus only seized the Rüsselsheim fortress and left Upper Hesse to Landgrave Georg. William's situation worsened when the king was killed in the 1632 Battle of Lützen.