On June 11th, Clarke sent to Goltz the information that the royalist spies in the Ministry of war had delivered via an officer that had left Paris on June 4th. The information about the Army was very accurate.
On this day, Napoleon made it clear when he planned to commence the campaign, pending any setbacks. Writing to Davout, he said:
My Cousin, you will inform Marshal Suchet, by courier and by telegraph, that hostilities will begin on the 14th, and that on that day he can seize Montmélian. If it is essential that he does so before this time, because of the movements of the enemy, then it is permitted. However it would be desirable that he did not do it before 15th.
So far, the Guard and VI Corps, the Corps that were in the interior, have begun marching to the frontier. IV Corps, the Army of the Moselle, has been called to the Army of the North, possibly to offset the troops in the Vendée, but also just as likely because Napoleon was about to attack two armies and was considerably outnumbered.
It is often stated that had Napoleon been able to use the troops dispatched to the Vendée he may have won the campaign. However, I would argue that if those numbers, around 10,000 depending on how many he would have taken presuming some would have had to stay behind, would have made the difference, it would only have been in a Pyrrhic victory. A decisive victory by maneuver was possible, and at this stage in 1815, I believe it was necessary. The Army of the North was the cream of Napoleon's forces, it constituted the mobile army he would use if France was later invaded. This campaign was largely a political one - seize Brussels, galvanize the popular support at home, stabilize his government, and push the King off the continent. As has been documented, Ghent was causing Napoleon a lot of problems.
The traitors could only desert once, and so this campaign could also be cleansing.
If Napoleon could quickly concentrate and strike before the Allies could consolidate their forces, then Napoleon felt they would be compelled to retreat, and Brussels could have been occupied with minimal loss. If either allied army chose to fight, and do so in isolation, Napoleon would have a tremendous advantage.
Thus, one can see why it was of paramount importance to get in position quickly and commence hostilities. Delays could, and would, be lethal.
The Allies were aware of a lot of French movements. But this should not be misunderstood. The frontier was not a static quiet place that suddenly erupted with activity. Wellington wrote Blücher on April 23rd stating that the French forces on the border were in perpetual motion. There was frequent desertion (soldiers were paid 20 francs, cavalry troopers 80). There were frequent false alarms, and the Allies could not and were not going to consolidate their armies on a particular route of advance by virtue the intelligence provided by civilians or those of the lower ranks. Their correspondence demonstrates as much was not believed as was believed. They would need compelling information, and in Wellington's case, proof of an actual advance in mass.
Further, Napoleon knew of the royalists and spies, and he certainly had his own. In fact, in his May 31st diary entry, Hobhouse wrote that he heard from Delort that there was a plan to separate Wellington and Blücher. At some point, Napoleon planted his own story of an advance via Mons after a feint on Charleroi. The advance through the Mons corridor had the advantages of striking at a single Allied army through more favorable terrain for attack. It was what Wellington feared most, and this rumor may have been a reason why Wellington was cautious at the beginning of the campaign.
Too often the analysis of the Waterloo campaign is done as though the participants knew the positions of the enemy as well as their own army perfectly - this was absolutely not the case, and will be a key part of the story during the hostilities.