It must be remembered that most of the soldiers in France at the time, including the ones sent to arrest Napoleon, were soldiers under napoleon for many years. So it's not unreasonable that the betrayed their king again to serve again under the man who had led them to victory dozens of times all across Europe. Despite the fact that Napoleon cared little for the lives of his men he knew how to speak to them, his 'common touch' inspired great loyalty in his soldiers combined with his numerous and extensive victory list resulted in near fanatical levels of devotion. (As can be seen in the Old Guard units.)

After Austerlitz when one of his Generals remarked on the numerous french dead lying on the ground Napoleon said

the women of Paris can replace those men in one night

in reply.

And also his reply to the Austrian Foreign minister in 1813 when he was offered honourable peace was to include a statement that he would 'Sacrifice a million men to achieve his ambitions.'

There were a good deal of Royal loyalists in the kings army however they were almost entirely confined to the officer class. The enlisted men were much more in favor of throwing their lot in with Napoleon. As can be seen with Count Alfred-Armand de Saint-Chamans who commanded the 7th Chassuers. As soon as he heard of Napoleons return he told his regiment to be ready for campaign,

'because I believed we were going to fight the ex-Emperor'.

His men had an entirely different objective however.

Someone told me that several officers had gathered in the cafe and were determined to take their troops to join the Light Infantry of the Guard to support the Emperor, that others were having tricolour flags made which they planned to give to the men and so provoke a mutiny... I began to see the true state of affairs and to feel the misery of my position. What could I do? Any hopes I had of giving the King a fine loyal regiment to support the throne at this fateful hour were dashed to the ground.

Colonel Girode de l'Ain was typical of many of the officers who had fought under Napoleon. With the return of the monarchy he had been forced to retire on half-pay and, though he was newly married, wanted to rejoin the Emperor as soon as he could. HE was living in the French Alps but decided he should go to Paris

The whole country was in turmoil. I traveled in uniform, but I took the precaution of providing myself with two cockades, one white and the other a tricolour, and depending on which colour flag I saw flying from the bell towers of any town or village we passed through, I quickly decorated my hat with the appropriate cockade.

What is happening hear is clear to see, the support for the present king Louis XVIII was very weak and his men were revolting beneath him. His military defected almost in its entirety to Napoleon whilst the King fled to the Belgian Border. Napoleon was forced to suppress Royalist unrest in Vendee which had always been a Royalist stronghold but in the grand scheme of things that was not really significant.

It's also worth noting that even Napoleon's Marshals were in the Royal service. In fact the very man in charge of the mission to arrest Napoleon was Marshal Michel Ney. The man who had fought under Napoleon through many years and many campaigns. Although he promised to bring Napoleon back to Paris in an Iron cage he instead defected his men, which is hardly surprising. These men were old friends, and reflected the attitude many soldiers had towards the issue. Napoleon had led them to victory and glory time and time again, he was the savior of France, conqueror of Europe and good to his men. That's a bit of hyperbole but it is reflective the attitude these men would have had.

In addition to military support Napoleon also possessed a great deal of public support. After all, it was only a few decades earlier that the French people staged their revolution and even before Napoleon reached Paris and enthusiastic crowd tore up the carpet of the Imperial Palace, removing the newly emblazoned crowns and revealing the Bee underneath. (One of Napoleons many symbols was the Bee.)

Sources

-Waterloo Bernard Cornwell

-Napoleonic Wars Charles Esdaile

-Waterloo Tim Clayton