On 1 March 1806, Deroy received the Grand Cross of the Military Order of Max Joseph. In the War of the Fourth Coalition, Deroy was placed under the command of Prince Jerome Bonaparte. He undertook several sieges of Prussian fortresses in Silesia during 1806 and 1807. In November 1806, Deroy's 1st Bavarian Division laid siege to the fortress of Głowgów (Glogau). After the Bavarians were replaced by a Württemberg division, the place surrendered on 2 December. He marched to Wrocław (Breslau) where his division arrived on 18 December. Prince Jerome's 22,000-strong IX Corps maintained the siege until 6 January 1807 when the fortress capitulated with 5,300 prisoners and 268 cannon. Bavarian casualties numbered 259 in the operation. Deroy then advanced with 13,000 troops to lay siege to Brzeg (Brieg). On 17 January Brzeg surrendered with 1,450 prisoners.