When Austria declared war on France and her allies, including Saxony, on 9 April 1809, a small force of Austrians together with a force of Brunswickers under the command of Frederick William, The Black Duke were based in Theresienstadt, Bohemia (now Terezín, Czech Republic) and acted as a defensive force whilst the bulk of the Austrian army was engaged in southern Bavaria and Italy. There was little action in the region until 25 May, when a Saxon force under Colonel von Thielmann invaded Bohemia. In response Duke Frederick William invaded Saxony and took the city of Zittau, forcing von Thielmann to retreat back to Dresden.