At Dead Carl and You, Kiran Pfitzner describes the disastrous 1806 campaign that knocked Prussia out of the war against Napoleon and shattered the military reputation of the army built by Frederick the Great:

Source – US Military Academy

In brief, the course of the 1806 campaign was that the Prussians met the French at Saalfeld and were initially defeated. The Prussians therefore decided to retreat before meeting Napoleon himself in battle. However, poor command organization and irresolution ended up dividing and delaying the Prussian forces. Thus, the rearguard ended up meeting the French main body under Napoleon which was able to overwhelm it at Jena.

The battle itself was not especially punishing, but the relentless pursuit of the French cavalry yielded many prisoners and prevented the reconstitution of the army. On the same day, the Prussian main body had encountered a French corps under Marshal Davout at Auerstedt, but failed to overcome it in a series of piecemeal attacks that cost it the lives of its commanders. The Prussians were demoralized enough that an attack from the outnumbered French was enough to force the main body into disorder. The arrival of fleeing forces from Jena spread a general panic and prevented any chance of recovery. From there, the campaign was a matter of pursuit and capitulation — within weeks the French were parading through the streets of Berlin. This humiliating defeat gave Clausewitz impetus to seek an understanding of the nature of war. How could the vaunted Prussian army, envy of the world in the days of Frederick the Great (still within living memory), be so summarily dispatched?

It was clear to virtually all military thinkers of the time that war had changed. To many, Napoleon was utilizing a higher, more perfect form of war than had been previously known. Clausewitz instead recognized that Napoleon was not refining war, but recognizing that changes in social conditions had enabled fighting with more energy and violence than had been possible in the cabinet wars of the 18th century. This had proven significant because the limitations of the 18th century made maneuver and logistics central to skilled generalship. Battles were important, but much that was won or lost in a battle could be subsequently lost or won outside it.

The removal of these restrictions drastically increased the importance of battle as it was able to produce results that could not be compensated for actions outside of it. Skilled generalship was therefore no longer a matter of outmaneuvering the enemy or protecting your supply lines while threatening his, but of bringing maximum force to the point of battle. Initiative, coordination, and aggression become the key traits of an officer. Thus, more expansively, the task of the officer is to recognize changes to the character of war and so understand what is required in practice. Neither history nor experience can anticipate these developments — it falls to the judgment of the individual to recognize them.

This framing shows clearly the mistakes of the Prussians. Operating in the old paradigm, they sought to make good with maneuver what they had lost in the opening battle of the campaign. They had divided their forces under the assumption the French would be unable or unwilling to aggressively pursue their retreat. At the same time, when they engaged the French, they showed caution entirely congruent with a cabinet army but fatally out of place when facing a Napoleonic force. On numerous occasions, the French made serious blunders that went unpunished because the Prussians failed to take the initiative and capitalize on them.

Nowhere is this more clear than in the failure of the Prussian main body to overcome the single corps it faced at Auerstedt. While outnumbering the French, the Prussian attacked piecemeal, becoming demoralized under French fire. The morale of the Prussians was substantially more brittle on account of their relative lack of nationalism — the state and therefore the army were not objects of any great affection by those subject to them. While this would require social reforms to remedy, Auerstedt had nevertheless been an opportunity for the Prussians. They had a French corps outnumbered more than two-to-one and merely needed to bring that force to bear to inflict a serious defeat. A Prussian victory would have positioned the main body to receive the retreating forces from Jena, allowing another confrontation with Napoleon on at least equal terms. Timidity and irresolution therefore played as big a part in the disaster as did the deeper defects.

In part, this must be ascribed to the advanced age of Prussian leadership. The senior commanders at both Jena and Auerstedt were over seventy. Not only did this ensure continuity with older forms of war, but men of such an age were unlikely to have the energy to campaign aggressively — by contrast, Napoleon and his marshals were three or four decades younger. The Prussian leaders did not lack physical courage, as their valiant deaths attest, but exposing oneself to danger is not the same quality that is needed for decisive and energetic action over an extended period of time.

The Prussian strategy deserves further criticism because by that point Bonaparte’s character was well known. There was no justification to have any illusions as to what the consequences of defeat would be. Prussia’s status amongst the great powers — if not its very existence as an independent state — would be determined by the confrontation. Leaving troops in Silesian fortresses or Polish garrisons (through which Prussia’s available forces were reduced by half) meant narrowing the odds of victory in pursuit of things that could be no substitute for victory and no comfort in defeat.