Mere days after Napoleon's bloody victory over the Russians at Drohiczyn, he received most unwelcome news, his reluctant ally, the Kingdom of Prussia had declared war on France. General Scharnhorst, who'd been been observing the battle, was sufficiently satisfied with the Russian's performance, rode a breakneck speed back to Berlin to inform his king. Frederick William III rather reluctantly made the official declaration of war in early June. Almost immediately, a massive portion of the Prussian army was tied down, due to the need to contain around 50,000 French and allied soldiers in the various fortresses of Prussia. In particular, General Rapp's newly designated XIV Corps in Danzig was well prepared for a long siege, even as Russian units moved to assist the Prussians, and the Duchy of Mecklenburg-Schwerin defected from Napoleon's Condederation of the Rhine.
Nonetheless despite having just two corps, about 30,000 men available in the field, General Blucher intended to aggressively march to the aid of Benningsen, who with around 60,000 men was facing a similar sized French force led by Marshal Masséna. Ten miles South of Stettin, Masséna had fallen on Bennigsen just as he'd crossed the Oder, their forces meeting on the 8th of June. At first the French had the better of it, Bennigsen was forced to pull his men into a tight defensive perimeter to defend his river crossing, hoping to hold out until nightfall. Masséna kept up an intricate and well planned assault, inflicting heavy losses on the Russians.
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From a small rise behind the battlefield, Masséna observed the struggle between his own forces and the Russians. Marmont was pushing hard, urging his men into the fight. "Tell Marmont to get his horse artillery up front." He said to his chief of staff "I want Bertrand to launch a full assault in half an hour." An aide-de-camp suddenly caught the marshals eye, he looked spooked "Sir!" He saluted "It's the Prussians, they're right on top us!" "Shit..." Masséna muttered under his breath "How many, and how far?" "At least a full corps sir, probably more. They can't be more than two, three miles at most.
The marshal grunted in frustration, this was exactly what he'd feared. "Alright, alright, how long until sunset?" His chief of staff looked at his pocket watch "Should be two more hours sir." "Alright, I can work with that. New orders, all three corps are to begin retreating South. Once Marmont is to the South of the Russian I'll coordinate the rearguard with him. This is going to be bloody."
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Despite the surprise of Bluchers attack, achieved in large part due to the French lack of cavalry to scout ahead, Masséna reacted quicky. While Bertrand and Lauriston's corps immediately marched to the south, Marmont's II Corps rapidly set up a defensive position. Bennigsen, not realizing how close Blucher's forces had come, was slow to organize a counterattack. While Blucher did launch a cavalry charge supported by infantry against Marmont, Masséna carefully coordinated a responce, and despite high casualties the French were able hold out until nightfall.
Despite the battle ending in a French defeat, Masséna's army was intact, and inflicted 12,000 Russian and Prussian casualties to just 8,000 of his own. As the retreat south to cover Dresden began, Napoleon and the main body of his army found themselves in an awkward position in Poland. Despite achieving victory against the Russians, and killing their top general, the race was on to escape Poland before they were trapped there. The French passed through Warsaw on the 8th of June, the Russians occupying the city two days later. Despite orders to behave, many Russian soldiers, Cossacks in particular, partook in looting, raping, and even murder. The officers were eventually able to resotore order, but not before almost 100 Polish civilians were murdered in the chaos. The war of the Sixth Coalition, had officially begun...