Late in the evening of June 14th a party of riders led by a Prussian staff officer appeared at the bridge over the Meuse at Namur demanding to see Blücher. Initially denied, the party insisted claiming to have with them a French General! The sentry believed the claim of escorting a French general to be an exaggeration in order to gain an audience, but relented in letting them pass. As Blücher had already retired, Gneisenau met with the party.
Whoever these riders were, the intelligence they brought was explosive. By midnight, Gneisenau had dispatched orders for the concentration of the Prussian army. II Corps was to concentrate on Mazy, III Corps on Namur,
and the IV Corps on Hannut. From these points, they would be further ordered to Sombreffe. Significant troop movements would now take place at least 12 hours earlier than they would have otherwise.
Zieten’s I Corps was braced for the attack, and would fall back on Fleurus if necessary.
Writing about the Prussian concentration, General Oscar Lettow-Vorbeck, authoring the German General Staff’s official history of the 1815 campaign, wrote:
Indeed, these orders made it possible to unite three Army Corps at Sombreffe, on the 16th, still before Napoleon’s attack. The IV. Corps could also have arrived there on time, had the order it had received been executed. However, it must not be overlooked that this was only possible because of the special messages received in the night leading up to the 15th. If the orders had only been given on the 15th, at 9 AM, after receipt of Zieten’s first message, a timely gathering so far frontwards would have been impossible and would have had to happen further behind. Without this treason committed by members of the French army, the surprise intended by Napoleon would have been successful to an even stronger degree than was the case now.
In another post, I will go into detail on the identity of these traitors.
If one looks at the map, and if one remember's Napoleon's planted rumor of an advance via Mons, one can see how easily it would have been for the Anglo-Dutch army to have a large contingent concentrated in the vicinity of Mons. Napoleon would remain very concerned about this possibility through all of June 15th and most of June 16th with consistent orders to his left to keep a strong eye to the west. His dispositions after crossing the Sambre were also made in a manner where the left could have been reinforced if attacked - he did this despite expecting and believing the Anglo-Dutch army was retreating.
One area of commentary about this campaign is that Napoleon was off - lazy, indifferent, etc. Further he is accused of arrogance and taking his opponents too lightly. However, having read all the correspondence I could find, I see someone who was diligent and accounted for all scenarios and was not ruled by his assumptions. His instructions were detailed, and when giving orders for maneuvers that would take place in his absence, he suggested the utmost caution. This attention to detail is an important element, for we'll soon see that one has to believe Napoleon did not care about detail to believe Napoleon was not betrayed.