Gneisenau allowed the re-establishment of his communications to overweigh the paramount necessity of arranging concerted action with his ally. Probably Wellington's failure to co-operate at Ligny had heightened the Prussian chief-of-staff's unworthy suspicions of the duke's good faith. It was well for the allies that Blucher was able to resume command before Napoleon had time to profit from the dissensions that would probably have arisen had Gneisenau remained in control. The casualties at Ligny were very heavy. The Prussians lost 12,000 men and 21 guns, and the French 8,500 men. So close was the fighting that most of the 20,000 casualties lay on 2 sq. m. of ground.
Napoleon's plan of campaign had succeeded. Despite D'Erlon's misadventure, Ney's failure had placed the Anglo-Dutch army in a precarious position. Napoleon having beaten Blucher, the latter must fall back to rally and re-form. On the other flank Ney lay in front of Wellington, and the marshal could fasten upon the Anglo-Dutch army and hold it fast on June 17, sufficiently long to allow the emperor to close round its open left flank and deal it a death-blow. It was essential to deal with Wellington before B1licher could re-appear on the scene. Wellington was but im perfectly informed of the details of the result of Ligny. Certainly Blucher had despatched an aide-de-camp to warn Wellington that he was forced to retire. But the officer was shot and the message remained undelivered. Nor did Gneisenau repeat this important message directly he assumed temporary command. Gneisenau's neglect involved the allies in an unnecessary and very grave risk.
June 17.—Napoleon was unwell, and was not in the saddle as early as he would otherwise have been ; and neither Soult nor Ney made any serious arrangements for an advance when every minute was golden. By early morning the duke had most of his army about Quatre Bras. But Bliicher's defeat had rendered Wellington's position untenable. Still ignorant of Blilcher's exact position, Wellington sent out a well-escorted officer to establish touch with the Prussians. He reported that the Prussians were drawing off to rally at Wavre. Then, about 9 A.M., a Prussian officer arrived to explain the situation and learn Wellington's plans. The duke replied that he should fall back and accept battle near Mt. S. Jean, provided he was assured of the support of one of Bliicher's corps. He now subordinated everything to remaining in communication with Blucher. It was 2 A.M., June i8, before Wellington received an answer.
Covered by Thielemann the Prussians had drawn off towards Gembloux to join Billow. Meanwhile, soon after dawn, the French cavalry rounded up some stragglers on the Namur road, and for a time confirmed the idea that Blucher was retiring on his base.
The situation was still obscure, details about what had happened to Ney were wanting, and the direction of the Prussian retreat was uncertain. At 8 A.M. Ney was ordered to take up his position at Quatre Bras, or if he reported that it was impossible the em peror would co-operate. Napoleon meant that if only a rear guard opposed Ney it was to be driven off and Quatre Bras occupied. But if Wellington was still there, the marshal was to hold him fast, and Napoleon would hasten up with the reserve and crush his enemy. Wellington in fact was there ; but Ney did nothing to retain him, and at io A.M. the duke began to retire northwards. The last chance of bringing about a decisive French success was thus allowed to slip away.
Grouchy's Operations.—About II A.M. Napoleon came to a decision. He determined to send two cavalry corps, and Van damme's and Gerard's corps, and Teste's division (33,00o and no guns) to follow the Prussians and discover if they intended uniting with Wellington in front of Brussels. As touch had been gained with Thielemann at Gembloux, Marshal Grouchy, who had been given command of the force, was ordered by the emperor to "proceed to Gembloux." This order the marshal obeyed literally. After an inconceivably slow march, in one badly arranged column moving on one road, Grouchy only reached Gembloux on June 17, and halted there for the night. Grouchy's cavalry who had been in touch with Thielemann's corps, at Gembloux, allowed it to slip away, and contact was lost for want of a serious effort to keep it. Grouchy did not proceed to the front and entirely failed to appreciate the situation. Pressing danger could only exist if Blilcher had gone northwards, and northwards in the Dyle valley Grouchy should have sought for the Prussians. But on June 17 the marshal pushed no reconnaissances to the northward and west ward of Gentinnes. (Actually Milhaud, when marching with Napoleon towards Quatre Bras, did see some Prussian infantry retiring northwards and reported this about 9 P.M. to Napoleon, but he attached little importance to it.) Had Bliicher gone east wards, then no danger threatened, for Grouchy could easily have held back any future Prussian advance on the line of the Dyle. Grouchy merely obeyed his orders literally and went to Gembloux. At nightfall the situation was in favour of the allies. The four Prussian corps were concentrated astride the Dyle at Wavre and Grouchy was actually outside them. After an unmolested retreat the Prussians were ready to take the field once more, and hours before Napoleon had deemed it possible after their defeat at Ligny.