"Impossible is not French."
(Attributed to Napoleon Bonaparte)
Background
Having conquered a large portion of western Europe since his coup d'etat, Napoleon Bonaparte, his plans to invade the island of Britain thwarted by diplomatic proceedings and the Battle of Trafalgar, turned to the east of Europe to face the combined forces of Russia and Austria in 1805. This third anti-French alliance, organised and largely financed by Great Britain to fight Napoleon, had been effectively destroyed at Austerlitz in the December of 1805. Prussia, however, was becoming increasingly belligerent and hostile towards France, and entered the war on the side of the coalition just before the battle of Austerlitz took place. It was, of course, too late for any Prussian forces to participate in this crushing defeat for the combined Austro-Russian force. The Prussians had formed an ultimatum to be put to the Emperor of France, one that would certainly and deliberately deteriorate relations between the two nations in order to provoke war. Prussia actually believed that they were militarily superior to France, primarily due to the legacy of Frederick the Great. As would become clear, however, Prussian military might had become seriously diminished by the 19th century.
Napoleon did not read the Prussian ultimatum until after his stunning victory at Austerlitz, although he had received it several days beforehand. Once he had read the its demands, however, the emperor took his revenge on Prussia by enforcing terms that the Germans were forced to accept, having no army mobilised at that time. The territories of Cleves, Ansbach and Neuchatel were therefore added to the ever-expanding French empire. The newly formed Confederation of the Rhine also threatened Prussian influence in the area, a humiliating blow. When Napoleon offered Hanover to Britain in return for an end to the hostilities in 1806, despite the fact that it was already promised to Prussia, the Prussian monarch, Queen Louise, would take no more. The Prussian army mobilised on the 10th of August and occupied the nearby province of Saxony (Sachsen). Saxony took Prussia's side once it became clear that she could not remain neutral.
So, by the autumn of 1806, war between France and Prussia was imminent. Napoleon moved just as swiftly as he had against Austria and Russia prior to Austerlitz, and his strategy was now to keep Prussia isolated from her two allies (Saxony and Russia). To do this he planned to strike deep into German territory, towards Dresden and Berlin. This is really one of the first examples of a blitkrieg, or "lightning war", demonstrating the military genius that was Napoleon Bonaparte. Such a bold move would also keep French forces close enough to the Austrian border to keep Austrian forces quiet (although after Austerlitz it was unlikely that they would play a significant role in the future campaign).
The Prussians, meanwhile, were slow in formulating a plan. Despite being well trained in contemporary tactics, the army lacked a leader in the same league as the French commander. Napoleon had succeeded in advancing so far that the Prussians actually had to turn and face the threat to their rear, as they too had moved forward in an attempt to cut Napoleon's lines of communication; but had been decicively out-maneouvred. It was only by the 5th of October, after weeks of procrastination, that this was even discovered. Prince Louis Ferdinand, one of the Prussian commanders, was killed in a skirmish between his forces and the French Corps commanded by Lannes at Saalfeld. This action was a mere shower compared to the imminent storm that was about to be unleashed upon the land by two of the mightiest armies in Europe.
On the 13th of October, the French V Corps, led by Marshal Lannes, discovered a Prussian force of around 30, 000 near the town of Jena (Iéna). The Emperor, who had expected to be fighting around Erfurt at least three days later, therefore ordered all his divisions to converge on the town by the 14th. Napoleon also intended that Marshal Davout and Marshal Bernadotte's Corps (who had been sent to cut off the Prussian line of retreat) would fall upon the enemy rear as a result of their advances from Naumburg and Dornburg respectively during the battle. Davout and his 29, 000 men therefore moved south, towards Apolda, and camped in a defile near Kösen late on the 13th. At the smae time Bernadotte interpreted his orders to mean that he should move more towards Dornburg, and chose a different route to that of Davout. This decision led to Bernadotte's Corps missing two of the greatest battles of the entire campaign, instead managing to march between them, apparently unaware of the carnage being caused mere miles away.
Meanwhile, however, the main Prussian force, under the Duke of Brunswick, was moving north, on a direct collision course with Marshal Davout and his men. En route to Halle, where it was supposed to meet the 15, 000-strong Prussian reserve, Brunswick's army consisted of 63, 000 soldiers, a much larger force than Davout had at his disposal. Arriving at Auerstädt on the night of the 13th in utter confusion, the Prussians failed to move towards the defile of Kösen (which the French had occupied in secret) and therefore left their right flank on the northwards march vulnerable.
As day dawned on the 14th of October 1806, neither side were aware of the presence of their enemy. it would take a chance encounter in the gloomy fog of that morning to kick start one of history's great battles.
The Battle
The first contact between the two forces was made by advance cavalry patrols from each side. Stumbling across an advance guard of Prussian horsemen in the thick morning fog at around 7am on the 14th, the French cavalry found themselves outnumbered. The French therefore retreated, allowing the infantry following up behind them to form squares in order to repel a possible cavalry charge. A major disadvantage for the French at Auerstädt was the lack of substantial cavalry forces of their own; although the Prussians would be grateful for this later in the day.
Leaving most of the infantry and artillery floundering in the rear, Blücher (the Prussian cavalry commander) ordered his troops to charge the right flank of the French positions at around 8am. Without artillery or infantry support the cavalry was unable to break the French squares, however, and was thouroughly routed by disciplined volleys from the Gallic infantry by around 8.30. The Prussian infantry continued to move up towards the French line during this action, although much of the army remained in confusion at Auerstädt.
By mid-morning this confusion had dissipated, allowing the majority of Prussian forces to deploy onto the field of battle. The main assault was directed against the village of Spielberg, which was fortunate for Davout. The French commander had placed two of his divisions in defence of Spielberg, leaving just one regiment (the 85th) around the nearby settlement of Hassenhausen. Fierce fighting erupted around both villages, however, and at one point Davout himself had to prevent the outnumbered and broken 85th from fleeing the battle altogether. The French line held, largely due to the piecemeal attacks made by the Prussians. Each German division advanced alone and isolated, allowing the French to destroy its fighting capability before it could be reinforced.
Events soon turned in the French's favour during the fight for Spielberg, as the Duke of Brunswick himself (the Prussian commander) was mortally wounded. The command of the army therefore fell to the Prussian king, Frederick-William II. He was inexperienced, however, and erroneously believed that he was facing the great Napoleon himself and was accordingly overawed by the situation. From this point on the Prussians lacked coherent command, although initially the king was resolute and vowed to stay and fight.
Davout received reinforcements at around 11am, and these proceeded to Hassenhausen to bolster the troops already there. Successive Prussian assaults against this village were again repulsed. By noon the French were ready to advance in turn, having inflicted heavy casualties on the enemy. Taking Spielberg and moving on towards Poppel resulted in a serious threat to the Prussian left, and repelling more cavalry charges (again, infantry in squares) further south left the right also open to the French. The news of defeat at Jena affected Prussian morale around this time, reducing their fighting effectiveness and really helping the French.
Frederick-William soon saw the need to withdraw rapidly, as his forces found themselves being surrounded. Prussian divisions under the command of Kalkreuth covered the retreat, but due to the lack of command and control it was fortunate for the Prussians that the French did not possess more extensive cavalry reserves. Only this prevented another complete rout of Prussian forces at Auerstädt.
The battle ended in early afternoon with the Prussian army fleeing the field, but with the French too exhausted by the battle and preceding marches to give effective pursuit.
Aftermath
The French had inflicted a massive 15, 000 casualties (dead and wounded) on the Prussians, in return for 7, 000 of their own (a large number relative to the size of Davout's force). 3, 000 prisoners and 115 artillery pieces were also captured by the French.
The Prussian army dissolved into chaos that night as they attempted to retreat back into Prussia and collided with their own forces who had been defeated at Jena. Most of the artillery and baggage had been abandoned during the movements of the night of the 14th. By the early hours of the 15th most of the army were far away from the day's battlefield.
Meanwhile the French, too exhausted to give chase, advanced slowly. Davout remained at Auerstädt over the night of the 14th/15th. Napoleon stayed the night at Jena, not yet having received Davout's dispatch concerning his action that day.
Looking at the big picture, all that remained of the Prussian armies after the double victory of Jena-Auerstädt were remnants scattered across the country. The remainder of the Jena Campaign for the French was essentially to mop up any remaining resistance, and by the 27th of October 1806 they were at the gates of Berlin. The French army entered with Davout, the hero of Auerstädt, at its head.
The 14th of October was a black day for Prussia - almost all of Prussia was now part of the French empire, and it directly led to the Treaty of Tilsit (signed in July 1807) which halved her size. However, forces to the east of the Oder river continued the fight, holding out in hope of their Russian allies coming to their aid, which they subsequently did, moving to the border with Prussia. Napoleon was about to make a monumental error in attempting to suppress them, one that would finally turn the tide against him. A mighty precedent would be set, one that Adolf Hitler ignored to his peril, in the invasion of Russia in the terrible Winter Campaign of 1806/07. From then on the French were forced to retreat all the way back to western Europe. Thus was the road to Leipzig and Waterloo made slick with the blood of those who fell at Jena and Auerstädt.
"On our right, Marshal Davout's Corps performed wonders. Not only did he contain, but drove back and defeated, for over three leagues, the bulk of the enemy's troops, which were to have debouched through Kosen. This marshal displayed distinguished bravery and firmness of character, the first qualities in a warrior."
(Bulletin published by Napoleon in recognition of Davout's victory at Auerstädt)
Sources:
"Jena 1806: Napoleon Destroys Prussia", by D. Chandler (published 1993)
"The Jena Campaign 1806", by F.N. Maude (first published 1909, 1998 edition)
http://www.fortunecity.com/victorian/riley/787/Napoleon/1806/1806.html
http://napoleon.musketwars.com/cmpgn/CPN_1806_Jena4.html#010
http://www.napoleonseries.org/reference/military/organization/artillery.cfm