The Hundred Days (French Cent-Jours) or the Waterloo Campaign commonly refers to the period between 20 March 1815, the date on which Napoleon Bonaparte arrived in Paris after his return from Elba, and 8 July 1815, the date of the restoration of King Louis XVIII. The phrase Cent jours was first used by the prefect of Paris, the comte de Chabrol, in his speech welcoming the King.
This period is also referred to as the War of the Seventh Coalition, since it was the seventh time in the French Revolutionary Wars and Napoleonic Wars that a combination of European countries had allied against France. This was the last conflict in the Napoleonic Wars, and was fought by a Coalition of Britain, Russia, Prussia, the Netherlands, Sweden, Austria, Spain, Portugal, Sardinia, and a number of German States against France. The Allied Coalition powers in Vienna declared Napoleon an outlaw and not the leader of France. It should be noted that the last fortress to surrender under seige was in September of 1815 thus it was not even a period that covered 100 days.
Exile in Elba
Napoleon spent nine months and 21 days in uneasy retirement on Elba (1814–1815), watching events in France with great interest. As he foresaw, the shrinkage of the great Empire into the realm of old France caused infinite disgust, a feeling fed every day by stories of the tactless way in which the Bourbon princes treated veterans of the Grande Armée. Equally threatening was the general situation in Europe. The demands of Czar Alexander I were for a time so exorbitant as to bring the powers at the Congress of Vienna to the verge of war. Thus everything portended a renewal of Napoleon's activity. The return of French prisoners from Russia, Germany, Britain and Spain would furnish him with an army far larger than that which had won renown in 1814. So threatening were the symptoms that the royalists at Paris and the plenipotentiaries at Vienna talked of deporting him to the Azores or to Saint Helena, while others more than hinted at assassination.
Return to France
Napoleon solved the problem in characteristic fashion. On 26 February 1815, when the British and French guardships were absent, he slipped away from Portoferraio with some 600 men and landed near Antibes on 1 March 1815. Except in royalist Provence, he received everywhere a welcome that attested to the attractive power of his personality and the nullity of the Bourbons. Firing no shot in his defence, his little troop swelled until it became an army. On 5 March, the 5th Infantry Regiment went over to Napoleon. The next day they were joined by the 7th Infantry Regiment under its colonel Charles-Angélique-François Huchet de la Bedoyère, who would be executed by the Bourbons for treason after the campaign ended. Ney, who had said that Napoleon ought to be brought to Paris in an iron cage, joined him with 6,000 men on 14 March; five days later the emperor entered the capital, whence Louis XVIII had recently fled.
An old anecdote illustrates either Napoleon's charisma or popularity, or (if untrue) the propaganda that operated in his lifetime and ever since: his army was confronted by troops sent by the king to stop him; the men on each side formed into lines and prepared to fire. Before fighting began, Napoleon walked between the two forces, faced the king's men, ripped open his coat and said "If any of you will shoot your Emperor, shoot him now." The men supposedly all joined his cause.
Napoleon was not misled by the enthusiasm of the provinces and Paris. He knew that love of novelty and contempt for the gouty old king and his greedy courtiers had brought about this bloodless triumph; and he felt instinctively that he had to deal with a new France, which would not tolerate despotism. On his way to Paris, he had been profuse in promises of reform and constitutional rule. It remained to make good those promises and to disarm the fear and jealousy of the great powers.
Return to power
This was the work he set before himself in the Hundred Days. One may doubt whether his powers, physical as well as mental, could equal the task. Certainly the evidence as to his health is somewhat conflicting. Carnot, Pasquier, Lavalette Thiéhault and others thought him prematurely aged and enfeebled. Others again saw no marked change in him; while Mollien, who knew the emperor well, attributed the lassitude which now and then came over him to a feeling of perplexity caused by his changed circumstances. This explanation seems to furnish a clue to the likely truth. Napoleon felt cramped and chafed on all sides by the necessity of posing as a constitutional sovereign; and, while losing something of the old rigidity, he lost very much of the old energy, both in thought and action. His was a mind that worked wonders in well-worn grooves and on facts that were well understood. The necessity of devising compromises with men who had formerly been his tools worried him both in mind and body. But when he left parliamentary affairs behind and took the field, he showed nearly all the powers of initiative and endurance that had marked his masterpiece, the campaign of 1814.
To date his decline, as Chaptal does, from the cold of the Moscow campaign is not fully correct. The time of lethargy at Elba seems to have been more unfavourable to his powers than the cold of Russia. At Elba, as Sir Neil Campbell noted, he became inactive and proportionately corpulent. There, too, as sometimes in 1815, he began to suffer intermittently from bowel problems, but to no serious extent. On the whole it seems safe to assert that it was the change in France far more than the change in his health which brought about the manifest constraint of the emperor in the Hundred Days. His words to Benjamin Constant -"I am growing old. The repose of a constitutional king may suit me. It will more surely suit my son"- show that his mind seized the salient facts of the situation, but his instincts struggled against them. Hence the malaise both of mind and body.
The royalists gave him little concern: the duc d'Angoulême raised a small force for Louis XVIII in the south, but at Valence it melted away in front of Grouchy's command; and the duke, on 9 April 1815, signed a convention whereby they received a free pardon from the emperor. The royalists of the Vendée moved later and caused more trouble. The chief problem was the constitution. At Lyon, on 13 March 1815, Napoleon issued an edict dissolving the existing chambers and ordering the convocation of a national mass meeting, or Champ de Mai, for the purpose of modifying the constitution of the Napoleonic empire. That work was carried out by Benjamin Constant in concert with the emperor. The resulting Acte additionel (supplementary to the constitutions of the empire) bestowed on France an hereditary chamber of peers and a chamber of representatives elected by the "electoral colleges" of the empire, which comprised scarcely one percent of the citizens of France. As Châteaubriand remarked, in reference to Louis XVIII's constitutional charter, the new constitution — La Benjamine, it was dubbed — was merely a slightly improved charter. Its incompleteness displeased the liberals; it garnered only 1,532,527 votes in the plebiscite, less than half of those of the plebiscites of the Consulate.
Not all the gorgeous display of the Champ de Mai (held on 1 June 1815) could hide the discontent at the meagre fulfilment of the promises given at Lyon. Napoleon ended his speech with the words "My will is that of the people: My rights are its rights." The words rang hollow. Napoleon was with difficulty dissuaded from quashing the 3 June election of Lanjuinais, the staunch liberal who had so often opposed the emperor, as president of the chamber of deputies. In his last communication to them, Napoleon warned them not to imitate the Greeks of the later Empire, who engaged in subtle discussions when the ram was battering at their gates. At Saint Helena he told Gourgaud that he intended in 1815 to dissolve the chambers as soon as he had won a great victory.
In point of fact, the sword alone could decide his fate, both in internal and international affairs. Neither France nor Europe took seriously his rather vague declaration of his contentment with the role of constitutional monarch of France. No one believed that he would be content with the "ancient limits". So often had he declared that the Rhine and the Netherlands were necessary to France that everyone looked on his present assertions as a mere device to gain time. As far back as 13 March, six days before he reached Paris, the powers at the Congress of Vienna declared him an outlaw; four days later the United Kingdom, the Netherlands, Russia, Austria and Prussia bound themselves to put 150,000 men into the field to end his rule. Their recollection of his conduct during the congress of Châtillon was the determining fact at this crisis; his professions at Lyon or Paris had not the slightest effect; his efforts to detach Austria from the coalition, as also the feelers put forth tentatively by Fouché at Vienna, were fruitless. The coalitions, once so brittle as to break at the first strain, had now been hammered into solidity by his blows. If ever a man was condemned by his past, Napoleon was so in 1815.
Napoleon knew that, once his attempts at dissuading one or more of the Seventh Coalition armies from invading France had failed, his only chance of remaining in power was to attack before the coalition put together an overwhelming force. If he could destroy the existing coalition forces in the Southern Netherlands before they were reinforced, he might be able to drive the British back to the sea and knock the Prussians out of the war. This was a successful strategy he had used many times before.
The state of French forces
Upon assumption of the throne, Napoleon found that he was left but little by the Bourbons and that the state of the Army was 56,000 troops of which 46,000 were ready to campaign.  By the 1st of June this was raised to 198,000 with 66,000 more in depots training up but not yet ready for deployment. 
The state of the opposition
Archduke Charles gathered Austrian and allied German states, while Schwartzenberg formed another Austrian army. King Ferdinand of Spain summoned English officers to lead his troops against France. Tsar Alexander mustered an army of 250,000 troops and sent these rolling towards the Rhine. Prussian Mustered two armies one under Blucher that sat aside Wellington's British army and its allies. The other was Prussian and North German allies under General Kleist. 
Napoleon moved two armies, the Army of the North (AotN) and the Reserve Army (RA) 128,000 men, up to the French Belgium frontier without alerting the coalition forces. He crossed the frontier and split his AoTN in two. He took the RA and the right wing of the AotN and attacked the Prussians under the command of General Gebhard Leberecht von Blücher at the Battle of Ligny on 16 June 1815. Moving up to the frontier without alerting the Coalition, Napoleon divided his army into a left wing, commanded by Marshal Ney, a right wing commanded by Marshal Grouchy and a reserve, which he commanded personally (although all three elements remained close enough to support one another). Crossing the frontier at Thuin near Charleroi, the French drove in Coalition outposts and secured Napoleon's favoured "central position" - at the junction between Wellington's Allied army to his north-west, and Blücher's Prussian to his north-east. Wellington had expected Napoleon to try to envelop the Allied armies by moving through Mons to the west of Brussels. Napoleon encouraged this view with false intelligence. A message from Wellington's intelligence chief, Sir Colquhoun Grant, was delayed by General Dörnberg, and Wellington first heard of the capture of Charleroi at 3p.m. shortly followed by another message from the Prince of Orange. Wellington ordered his army to collect at their divisional headquarters, but was still unsure whether the attack in Charleroi was a feint and the main assault would come from Mons, and Wellington only found out with certainty Napoleon's intentions and sent out orders for the mustering of his army near Nivelles and Quatre Bras just before midnight on the 15 June. 
The Prussians were not taken unaware and began to concentrate, as Napoleon considered the Prussians the greater threat, he moved against them first, attacking their outposts at Thuin near Charleroi, before advancing through Charleroi. His scouts reached Quatre Bras that evening. Ziethen's 1st Corps rearguard action held up Napoleon's advance, giving Blücher the opportunity to concentrate his forces in the Sombreffe position, which had been selected earlier for its good defensive attributes. Napoleon sent Marshal Ney, in charge of the French left, to secure the crossroads of Quatre Bras, towards which Wellington was hastily gathering his dispersed army. Once Quatre Bras was secured, Ney could swing east and reinforce Napoleon.
Ney, advancing on 16 June, found Quatre Bras lightly held by Allied troops, but having previously experienced Wellington's skill at concealing his strength, he overestimated the forces opposing him. Despite outnumbering the Allies heavily throughout the day, he fought a cautious and desultory battle which failed to capture the crossroads. By the middle of the afternoon, Wellington had taken personal command of the Allied forces at Quatre Bras. The position was reinforced steadily throughout the day as Allied troops converged on the crossroads. Finally, they were able to advance and drive the French back.
Napoleon, meanwhile, took the reserve and the right wing of the army and defeated the Prussians, under the command of General Blücher, at the Battle of Ligny on the same day. The Prussian centre gave way under heavy French attack, but the flanks held their ground. Had Ney intervened at this point as planned, the Prussians would have been partially encircled from the west and would almost certainly have been forced to fall back to the east, along their lines of communication.
The Prussian defeat at Ligny made the Quatre Bras position untenable. On 17 June Wellington duly fell back to the north. His control of Quatre Bras enabled the Prussians to fall back parallel to his line of retreat and not, as Napoleon had hoped, away from him.
This was part of Napoleon's strategy to split the much larger Coalition force into pieces that he could outnumber and attack separately. His theory was based on the assumption that an attack through the centre of the Coalition forces would force the two main armies to retreat in the direction of their respective supply bases, which were in opposite directions.
The general retreat of the Prussian army took it to the town of Wavre, and this by default became the marshalling point of the army. The Prussian chief of staff, General August von Gneisenau, planned to rally the Prussian Army at Tilly, from where it could move to support Wellington, but control was lost, with part of the army retreating towards the Rhine, but the majority rallied at Wavre. General Blücher arrived at Wavre - having fallen under his horse whilst leading a counter charge, and then been ridden over by French cavalry twice - and after a meeting Gneisenau was persuaded to march upon Wellington's left flank at dawn with the I, II and IV Corps. The IV Corps, under the command of General Bülow von Dennewitz, had not been present at Ligny, but arrived to reinforce the Prussian army during the nights of the 17th and 18th. III Corps formed the rearguard, to hinder the pursuing French.
On the morning of 17 June Napoleon sent the right wing of the Army of the North under the command of Marshal Grouchy to harass the Prussians to stop them reforming. He set off via Quatre Bras with the RA and combined his forces with the left wing of the AotN to pursue Wellington's forces, which were retreating towards Brussels. Just before the small village of Waterloo, Wellington deployed most of his forces on the rear side of an escarpment. He placed some of his forces in front of the main deployment in two fortified farmhouses at the base of the escarpment, which guarded the two roads to Brussels.
It was here on 18 June 1815 that the decisive European battle of the 19th century took place. The start of the Battle of Waterloo was delayed for several hours as Napoleon waited until the ground had dried from the previous night's rain. By late afternoon the French army had not succeeded in driving Wellington's allied forces from the escarpment on which they stood. Once the Prussians arrived, attacking the French right flank in ever increasing numbers, Napoleon's key strategy of keeping the Seventh Coalition armies divided had failed and his army was driven from the field in confusion, by a combined coalition general advance.
Within the sound of cannon fire from Waterloo a second battle took place at the village of Wavre. Grouchy, who was dilatory in his pursuit of the Prussians, failing to stop them regrouping after their defeat at Ligny, attacked the Prussian III Corps under the command of General Johann von Thielmann, believing that he was engaging the rearguard of a still-retreating Prussian force. However only one Corps remained — the other three Prussian Corps (I, II and the still fresh IV) were marching towards Waterloo.
The next morning the battle of Wavre ended in a hollow French victory. Grouchy's wing of the Army of the North withdrew in good order and other elements of the French army were able to reassemble around it. However, the army was not strong enough to resist the combined coalition forces, so it retreated towards Paris.
End of Napoleon's career
On arriving at Paris, three days after Waterloo, Napoleon still clung to the hope of concerting national resistance; but the temper of the chambers and of the public generally forbade any such attempt. Napoleon and his brother Lucien Bonaparte were almost alone in believing that, by dissolving the chambers and declaring Napoleon dictator, they could save France from the armies of the powers now converging on Paris. Even Davout, minister of war, advised Napoleon that the destinies of France rested solely with the chambers. Clearly, it was time to safeguard what remained; and that could best be done under Talleyrand's shield of legitimacy. Napoleon himself at last recognised the truth. When Lucien pressed him to "dare", he replied "Alas, I have dared only too much already". On 22 June 1815 he abdicated in favor of his son, Napoléon Francis Joseph Charles Bonaparte well knowing that it was a formality, as his son was in Austria. On 25 June he received from Fouché, the president of the newly appointed provisional government, an intimation that he must leave Paris. He retired to Malmaison, the former home of Josephine, where she had died shortly after his first abdication. On 29 June the near approach of the Prussians, who had orders to seize him, dead or alive, caused him to retire westwards towards Rochefort, whence he hoped to reach the United States. The full restoration of Louis XVIII followed the emperor's departure.
- Chesny, Charles A: Waterloo Lectures:A Study Of The Campaign Of 1815 ISBN 1428649883