The Prussians did not know the man with whom they had to deal. It was an idle hope that this state could cope alone with the power of Napoleon and his allies, and while Frederick William was slowly preparing for the war which he had long sought to avoid, the French troops were on the march and rapidly approaching the borders of his kingdom. Saxony had allied itself with Prussia under compulsion, and had added 20,000 men to its armies. The elector of Hesse-Cassel had also joined the Prussians, and furnished them a contingent of troops. But this hastily levied army, composed of men few of whom had ever seen a battle, seemed hopeless as matched with the great army of war-worn veterans which Napoleon was marching with his accustomed rapidity against them. Austria, whom the Prussian King had failed to aid, now looked no passively at his peril. The Russians, who still maintained hostile relations with France, held their troops immovable upon the Vistula. Frederick William was left to face the power of Napoleon alone. [=March of the French Upon Prussia=] The fate of the campaign was quickly decided. Through the mountain passes of Franconia Napoleon led his forces against the Prussian army, which was divided into two corps, under the command of the Duke of Brunswick and the Prince of Hohenlohe. The troops of the latter occupied the road from Weimar to Jena. The heights which commanded the latter town were seized by Marshal Lannes on his arrival. A second French corps, under Marshals Davout and Bernadotte, marched against the Duke of Brunswick and established themselves upon the left bank of the Saale. On the morning of the 4th of October, 1806, the conflict at Jena, upon which hung the destiny of the Prussian kingdom, began. The troops under the Prince of Hohenlohe surpassed in number those of Napoleon, but were unfitted to sustain the impetuosity of the French assault. Soult and Augereau, in command of the wings of the French army, advanced rapidly, enveloping the Prussian forces and driving them back by the vigor of their attack. Then on the Prussian center the guard and the reserves fell in a compact mass whose tremendous impact the enemy found it impossible to endure. The retreat became a rout. The Prussian army broke into a mob of fugitives, flying in terror before Napoleon's irresistible veterans. [=Defeat of the Prussians at Jena and Auerstadt=] They were met by Marshal Biechel with an army of 20,000 men, advancing in all haste to the aid of the Prince of Hohenlohe. Throwing his men across the line of flight, he did his utmost to rally the fugitives. His effort was a vain one. His men were swept away by the panic-stricken mass and pushed back by the triumphant pursuers. Weimar was reached by the French and the Germans simultaneously, the former seizing prisoners in such numbers as seriously to hinder their pursuit.