Sickness and desertion had begun to diminish the army's numbers as the weather turned bad.

The nominal army commander, Archduke Ferdinand, and Mack's chief of staff General-Major Anton Mayer von Heldensfeld, had both insisted that the army halt at the Lech as originally planned.

By the end of September, relations between Mack and Ferdinand had become so poor that all communication between the two was done in writing.
 
Ferdinand and Mayer had appealed to Emperor Francis II.

The emperor had sought the advice of Feldmarschall Archduke Charles, who commands the Army of Italy, and had been warned that Mack is making a strategic blunder.

Even so, the emperor had backed Mack to the hilt and relieved Mayer of his post.

Mack's army begins to assemble on the Iller.

On 24 and 25 September, Napoleon had launched the Grande Armée across the Rhine River to open the Ulm Campaign.

While Marshal Joachim Murat's Cavalry Corps and Marshal Jean Lannes's V Corps advances directly east toward Ulm, the bulk of Napoleon's army passes to the north of the Austrian army.

Marshal Jean-Baptiste Bernadotte's I Corps, General of Division Auguste Marmont's II Corps, Marshal Louis Davout's III Corps, Marshal Nicolas Soult's IV Corps, and Marshal Ney's VI Corps had wheeled east, then southeast, then south.

On October 5, Kienmayer had reported that the French were in Ansbach, to the north of the Danube.

Two days later, the French cross the Danube on a broad front, moving south.

At this time Mack's army is divided into four corps.

Jellačić has fifteen thousand troops in 16 infantry battalions, six Jäger companies, and six cavalry squadrons to the south of Ulm.

Feldmarschall-Leutnant Karl Philipp, Prince of Schwarzenberg, commands twenty-eight battalions and thirty squadrons at Ulm.

Feldmarschall-Leutnant Franz von Werneck has thirty battalions and twenty-four squadrons near Günzburg.

Kienmayer's command near Ingolstadt consists of nineteen battalions and  thirty-four squadrons.