Bluecher’s army, comprising 90,000 men, also covered a great stretch of country.  The first corps, that of Ziethen, held the bridges of the Sambre at and near Charleroi; but the corps of Pirch I. and Thielmann were at Namur and Ciney; while, owing to a lack of stringency in the orders sent by Gneisenau, chief of the staff, to Buelow, his corps of 32,000 men was still at Liege.  Early on the 15th, Pirch I. and Thielmann began hastily to advance towards Sombref; and Ziethen, with 32,000 men, prepared to hold the line of the Sambre as long as possible.  His chief of staff, General Reiche, states that one-third of the Prussians were new troops, drafted in from the Landwehr; but all the corps gloried in their veteran Field-Marshal, and were eager to fight.

Such, then, was the general position.  Wellington was unaware of his danger; Bluecher was straining every nerve to get his army together; while 32,000 Prussians were exposed to the attack of nearly four times their number.  It is clear that, had all gone well with the French advance, the fortunes of Wellington and Bluecher must have been desperate.  But, though the concentration of 125,000 French troops near Beaumont and Maubeuge had been effected with masterly skill (except that Gerard’s and D’Erlon’s corps were late), the final moves did not work quite smoothly.  An accident to the officer who was to order Vandamme’s corps to march at 2 a.m. on the 15th caused a long delay to that eager fighter.[478] The 4th corps, that of Gerard, was also disturbed and delayed by an untoward event.  General Bourmont, whose old Vendean opinions seemed to have melted away completely before the sun of Napoleon’s glory, rewarded his master by deserting with several officers to the Prussians, very early on that morning.  The incident was really of far less importance than is assigned to it in the St. Helena Memoirs, which falsely ascribe it to the 14th:  the Prussians were already on the qui vive before Bourmont’s desertion; but it clogged the advance of Gerard’s corps and fostered distrust among the rank and file.  When, on the morrow, Gerard rejoined his chief at the mill of Fleurus, the latter reminded him that he had answered for Bourmont’s fidelity with his own head; and, on the general protesting that he had seen Bourmont fight with the utmost devotion, Napoleon replied:  “Bah!  A man who has been a white will never become a blue:  and a blue will never be a white.”  Significant words, that show the Emperor’s belief in the ineradicable strength of instinct and early training.[479]

Despite these two mishaps, the French on the morning of the 15th succeeded in driving Ziethen’s men from the banks of the Sambre about Thuin, while Napoleon in person broke through their line at Charleroi.  After suffering rather severely, the defenders fell back on Gilly, whither Napoleon and his main force followed them; while the left wing of the French advance, now intrusted to Ney, was swung forward against the all-important position of Quatre Bras.