Believing Napoleon would fight defensively, the Prussian military command resolved to take the offensive as soon as possible, not waiting for their Russian allies. After much debate, the Prussians split their army, with one part advancing to the south and the other to the south-west. They concentrated their forces near Erfurt, hoping to catch the French before they mustered.
Unfortunately for the Prussians, Napoleon had no intention of fighting a defensive war. On the day the ultimatum was sent, Napoleon had already left Paris and was on his way to Würzburg. At the time of the war declaration, there were at least 160,000 French troops already stationed in southern Germany, which Napoleon quickly mobilised. He needed to defeat the Prussians before they were joined by their Russian allies, and for that to happen, he needed a quick line of advance to limit Prussia’s retreat options.
Unlike the Prussians, by mid-September, Napoleon had already decided on a plan. Following reports that the line of operations would take place along the River Main towards Würzburg, he rapidly concentrated his forces at Bamberg. Seeking to outflank the Prussians, the plan was then to advance north and then west via Leipzig and Berlin, cutting the Prussians off from Russian reinforcements. To achieve this, the French army would first have to cross the Franconian and Thuringian forest. Napoleon opted for the most flexible option: splitting his army into three columns, each powerful enough to deal with a Prussian detachment on their own but also close enough to come to each other’s aid if necessary, which would advance together in a strong, wedge-shaped formation.
On 8th September, the deadline for France to reply to the ultimatum, the French columns were crossing the forest. Whilst Bernadotte and Murat engaged and defeated Tauentzien’s Prussian forces at Schleiz, the other columns advanced on several Prussian positions including Hof and Saalfeld, where other minor skirmishes occurred. By 13th October, Lannes’ corps had caught sight of a large Prussian force, which Napoleon assumed to be main Prussian army. Napoleon subsequently sent the majority of his forces to Jena, while Bernadotte and Davout’s corps were sent to outflank to the north. This meant the Prussians were nearly encircled and had limited options to escape: ready or not, they had to fight.