At the beginning of hostilities Petrasch commanded a Division of Wallis’ 1st Corps under Charles on the Danube, and served at the Battle of Stockach 25 March. During the invasion of Switzerland in May he was detached to link with the Vorarlberg Corps of Hotze, his command becoming the reserve. He commanded the 4th Column in Hotze’s left wing at the First Battle of Zurich (Zurich Berg) 4 June, and temporarily replaced Hotze when that officer was wounded. During the Second Battle of Zurich after the death of Hotze on the Linth 25 September he replaced him as commander of the Vorarlberg Corps, then, learning of the defeat of Korsakov's Russians, retreated precipitously beyond the Rhine, effectively abandoning the Corps of Alexander Suvorov to suffer the full attentions of the French. Transferred to command the garrison of Ulm on the Danube in December, he was left behind at Ulm with 10,000 men when Kray retreated after the Battle of Höchstadt 19 June."