No sooner did Napoleon know that the Prussians had advanced into the heart of Saxony, than he formed the plan of his campaign:  and they, persisting in their advance, and taking up their position finally on the Saale, afforded him, as if studiously, the means of repeating, at their expense, the very manoeuvres which had ruined the Austrians in the preceding campaign.  In a word, he perceived that the Prussian army was extended upon too wide a line, and the consequent possibility of destroying it in detail.  He further discovered that the enemy had all his principal stores and magazines at Naumburg, to the rearward, not of his centre, but of his extreme right; and resolved to commence operations by an attempt to turn the flank, and seize those magazines, ere the main body of the Prussians, lying at Weimar, could be aware of his movement.  The French came forward in three great divisions; the corps of Soult and Ney, in the direction of Hof; Murat, Bernadotte and Davoust, towards Saalburg and Schleitz; and Lannes and Augereau upon Coburg and Saalfield.  These last generals were opposed sternly, at Saalfield, by the corps of Prince Louis of Prussia.  This brave young officer imprudently abandoned the bridge over the Saale, which he might have defended with success, and came out into the open plain, where his troops were overpowered by the French impetuosity.  He himself, fighting hand to hand with a subaltern, was desired to surrender, and replying by a sabre cut, was immediately struck down with a mortal thrust.  The Prussians fled; the bridge, which ought to have defended, gave the French access to the country behind the Saale.  The flank of the Prussian position was turned:  the French army passed entirely round them; Napoleon seized Naumburg, and blew up the magazines there,—­announcing, for the first time, by this explosion, to the King of Prussia and his Generalissimo, the Duke of Brunswick, that he was in their rear.

From this moment the Prussians were isolated, and cut off from all their resources, as completely as the army of Mack was at Ulm, when the French had passed the Danube and overrun Suabia.  The Duke of Brunswick hastily endeavoured to concentrate his forces for the purpose of cutting his way back again to the frontier which he had so rashly abandoned.  Napoleon, meantime, had posted his divisions so as to watch the chief passages of the Saale, and expected, in confidence, the assault of his outwitted opponent.  It was now that he found leisure to answer the manifesto of Frederick William, which had reached Paris a day or two after he himself quitted that capital for the camp.  His letter, dated at Gera, is written in the most elaborate style of insult.  The King of Prussia (said he) had sent him a silly pamphlet of twenty pages, in very bad French—­such a pamphlet as the English ministry were in the habit of commanding their hireling scribblers to put forth—­but he acquitted the King of having read this performance.