Late July, 1809
The long march into Saxony
After Victor's decisive victory and steadfast advance northward, things were proceeding smoothly for the French. Truth be told however, Victor had sent several messengers to inform the Emperor of his recent battle, but none had reached their target yet. This was due to multiple factors; for one, the weather had suddenly worsened and heavy rains started pouring on central Europe, plunging the already badly crafted roads of the countryside into muddy masses of yellowish grass, boulders and fallen trees. In such atmosphere, small bands of Prussian partisans had started to pick up any Frenchman in sight with even the slightest semblance of soldierly on their person, wether it be just a blue jacket or a bicorne that looked like it could belong to the old Grenadiers. No small patch of woodland was safe from these bandits, whose unscrupulous advance had engulfed progressively new territories. Napoleon's very own rearguard had suffered from their actions; as they approached Frankfurt, still concealed by a tight network of subterfuge, Davout himsellf risked a ruinous fall from his horse after one of these men tried to shoot him from a safe distance, just outside of a grassy field densely packed with tall bushes.
Although he had missed the mark, the man managed to run back in the woodland and hide from his pursuers, helped by the morning mist. Deeply irritated by this behaviour, Napoleon ordered his vanguard to scout deeper northeast, expecting a German contingent from Wurzburg soon. Berlin was still his top priority, although having now considered how the Prussians were playing their game, he fully intended to reach Konigsberg and make king Frederick Willhelm pay in full for his bravado.
Still unaware of Victor's accomplishment, the Emperor changed course and, instead of moving directly towards Berlin as previously planned, he stepped into Bamberg deploying his troops, previously in marching order, into six columns shielded on their flanks by eight cavalry squadrons and two voltigeur battalions. At the heart of this massive force his beloved artillery, among which the Old Guard artillery batteries were exercising their inspiring presence, was well guarded from all sides and ready to deploy at any moment, his men as eager to do battle as they have ever been. Overall morale was very high. For this very reason, the days that followed were marked by a sudden change of pace for kings and emperors alike: Saxony was in a deep state of turmoil, due to the sudden Anglo-Prussian invasion. Due to London and Konigsberg's jointed pressure, Saxon standing abruptly shifted towards Britain, wether it be aided by positive intentions or threats it's still open for debate. What is certain however, is that the Saxon troops stationed at the border begun a slow, steady retreat inland while the few Saxon infantry contingents already present in the French army, as soon as they caught wind of this, started arguing among themselves with increasing fervor. Nobody was completely surprised as the heart of the rebellion had expanded southward and both Leipzig and Dresden had been overrun by British and Prussian troops, but it was still unheard of that such a force alone could pose a formidable threat of this proportion. Aware of such movements, Napoleon sought to find more by issuing immediate orders to his forces still stationed in Saxony, of whose healthy condition he was assured of by multiple sources dating back more than a week before. Each garrison should resist occupation and provide support to one another by linking their respective lines of communication. This task would have provided Napoleon with a fully organized screen of skirmishers in and around Dresden, for he had previously raised new regiments of voltigeurs and chasseurs from his garrisons thanks to a very strong cadre of veterans from Boulogne, still standing among the finest in the French army. Fresh and inexperienced, but eager to demonstrate their valour, these young souls would very soon take arms and embrace their personal baptism of fire.
Luckily for the Emperor, two such forces had risen from Saaz and Breslau, and both were still unmolested. Bessieres hastly sought to send his messengers to both garrisons while the complex machine of Chappe telegraphs aided in the task; five more days later, Napoleon had a clearer picture of the situation at hand, but he didnt want to wait any longer. With Lannes' Corps of veteran troops approaching Frankfurt as well and Mortier's entering Hannover, the Emperor decided to launch a powerful attack on the hilly saxon plains, marching towards Leipzig. From there, after a swift victory, he had planned to march directly into Prussia and push as hard as he dared to capture Berlin with Bernadotte linking up with Victor.
However...
Victor was still waiting further orders, stuck in the muddy fields of Lower Bavaria. He had tried to pursue general von Schuler for a good while, but the incessant rain prevented him from capturing the remnants of the Prussian Korps. Still very satisfied with the results of his battle and the abundant loot taken in its aftermath, all he could do now was sending the captured troops home in honorable defeat and confiscate all the material available.
On a completely different turn of events, Bernadotte was comfortably watching a long stream of Prussian and British troops flooding the northern sector of Prague, partly concealed by unprincipled Austrian shopkeepers and local garrisons. He had sought an opportunity to contact an Austrian ambassador of his, a Karl whose name had been kept secret for several months, who had contacts within the British government and was apparently eager to exploit his bitter rivalry towards Napoleon as much as he allowed himself to.
Nobody knows with certainty which enticing promises he whispered to his soul, but the offer seemed great enough as to warrant his temporary loyalty. As such, the whole of his 60.000 men had been moving in circles for well over two weeks by then, under the pouring rain and ever growing supply problems. Issuing a sudden countermarch, Bernadotte's men were sent back towards Linz, from there they would reach Salzburg and backtrack to Ulm.
Now Napoleon's right flank was completely open to Prussian harassment and, with Victor's lateness, Lannes' rearguard could have been taken by surprise at any moment. Seeking such an exploit was the skilled Prussian general Moltke, whose organized resistance, mostly comprised of militia, lancers and a great deal of reserve troops from the years 1807-1808, had proven as fierce as that of a regular Korps. With the winds in his favour and aided by a heavy storm approaching southward, his men marched towards Frankfurt to intercept Lannes and cut him off from the rest of the French army. Little did they know that among his troops the Old Guard itself was ready to deploy.