At the beginning of the War of the Fifth Coalition, Deroy relinquished command of the 1st Division to Crown Prince Ludwig of Bavaria and took charge of the 3rd Division in the VII Corps. This unit consisted of the 5th, 9th, 10th, and 14th Infantry Regiments, the 5th and 7th Light Battalions, the "Taxis" Dragoons and "Bubenhoven" Chevau-légers, and 18 pieces of artillery. After the Austrian invasion, Deroy held the west bank of the Isar River opposite Landshut on 16 April 1809. In a brief action, each side lost 150 to 200 killed and wounded. After an Austrian column crossed upstream at Moosburg, he fell back through Pfeffenhausen to Siegenburg. Under the command of Marshal François Joseph Lefebvre, Deroy was present but not engaged in the Battle of Abensberg on 20 April.