In 1805, Jelačić commanded a corps in the army of Archduke Ferdinand Karl Joseph of Austria-Este and Karl Mack von Leiberich during the Ulm Campaign. At first his troops defended Biberach an der Riss. Around 6 October, Mack ordered Jelačić to move toward Ulm. At this time, Jelačić commanded 15,000 troops organized in 16 infantry battalions, six Jäger companies, and six cavalry squadrons. Emperor Napoleon I of France and his Grande Armée began to envelop the Austrian army. During the Battle of Wertingen on 8 October, the Battle of Günzburg on 9 October, and the Battle of Haslach-Jungingen on 11 October, the Grande Armée began to close in on its prey. On the 12th, Mack reorganized his army, making Jelačić one of four corps commanders, the others being Johann Sigismund Riesch, Franz von Werneck, and Karl Philipp, Prince of Schwarzenberg. He then ordered Jelačić to march south toward the Tyrol via Ochsenhausen for no explainable reason.