With the war of the Fourth Coalition starting in 1806 and being primarily fought in Germany, Poland and the Russian frontier, Forgues found himself serving in Marshal Ney's corps, which was detached from Murat's forces to besiege and obtain the surrender of the Prussian forces of General von Kleist that had sought refuge in Magdeburg. The siege lasted from 25 October to 8 November 1806, the Prussians capitulating and evacuating the fortress on 11 November 1806.