During the night of 25^th-26^th February 1815, Napoleon escaped from Elba, landing in France, despite the English blockade, crossing France on a wave of popular enthusiasm, supported by the army, twenty days later, he entered Paris, from which Louis XVIII had fled to Belgium (20^th March).

At the news of the return of Napoleon, the coalition took up arms. The Emperor immediately set about re-organising the army. The recall of old soldiers added 70,000 men to the 180,000 men on the strength of the 1^st March Napoleon called up the conscription of 1815, and in May he had 300,000 men under arms. He formed 5 army corps at Lille, Valenciennes, Mezieres, Metz, and Soissons. The 21eme was part of the 1^st Corps formed at Soissons under the command of Count d’Erlon. The Regiment commanded by Colonel Carre formed two battalions, with an effective strength of 45 officers and 952 men, was quartered at Lille in the month of April 1815, as part of Marcognet’s division.

5th May, the Regiment was dispatched to Valenciennes. Pressing orders were given to the depot to prepare for war all available men. The two war battalions received. On the 17^th May, a detachment of 59 men, the effective strength was brought up to 1,017 men. The 21^st May, the Regiment along with the 46^th Line, became part of Nogues’ 1^st Brigade, of Marcognet’s 3^rd Division, of the 1^st Army Corps.

In order that the depots would not be trapped in case the enemy passed the frontier, the Emperor gave the order to move them away from the northern frontier. In consequence the depot of the 21eme, which had been at Cambrai, was moved to St-Quentin at the end of May, and some days later it was moved to Riom (Puy-de-Dome), where it arrived on the 1^st July.

5^th June, the two battalions of the Regiment were dispatched to Valenciennes; on the 10^th there were at Quaroubles and Rombies.

14^th June, having passed Maubeuge, the 21eme took up a bivouac with the 1^st Corps, two leagues behind Solre-sur-Sambre. Fires were not allowed so as not to alert the enemy. The order of the day required the unit commanders to ensure that as well as the necessary munitions, the men had bread for four days and a half-pound of rice per man. In the evening Count d’Erlon received the order to put his Corps in motion the next day, at 3am, and to endeavour to gain as far as possible the left of the 2^nd Corps (Reille).

 The 1^st Corps passed Marchiennes and marched on Gosselies, by the Charleroi-Brussels road. The English being in this direction. The Emperor placed the first two corps d’Erlon and Reille under the orders of Marshal Ney, who were to be established at Quatre Bras. Ney stopped at Frasne on the 16^th, with Reille’s corps, with the intention of being supported by d’Erlon’s corps; but this latter, leaving Marchiennes late, let his men rest on the Charleroiroad. During this time, assuming that Ney had taken Quatre Bras, ordered Count d’Erlon to attack the reserves of the Prussian army at the Bry Windmill, following the Old Roman Road. Count d’Erlon, following the Emperor’s orders, began his movement on the 16^th at midday, to make this diversion. He had arrived at the heights of Viller Pernin when he received the urgent order from Marshal Ney to return to Quatre Bras. At about six in the evening, he changed direction to the left, marching on Frasnes along the wood. It was at this time that the Emperor was about to attack Ligny with the Imperial Guard. (This was delayed) by an enemy column debouching from the wood, turning the flank of the French Army. This column turned out to be d’Erlon’s Corps which manoeuvred in obedience to Marshal Ney’s orders. Ligny was carried; but at Quatre Bras, Ney’s tardiness allowed the union of the Anglo-Dutch army under the command of the Duke of Wellington. This latter, informed of the defeat of Blucher at Ligny, recrossed the Dyle on the 17^th, abandoning Quatre Bras. The French occupied the place.

 The 1^st Corps took the lead in the pursuing column along with the cavalry division of Jacquinot, arriving at seven in the evening, at the Maison du Roi, establishing bivouacs over (straddling) the main road between La Belle Alliance and Rossomme. The 1^st Corps was to attack Mont St Jean, on the 18^th, having taken the village, they were to put it into a state of defence. Towards 11am, on the 18^th, the moved to the right of the road from Charleroi, extending obliquely along the Smohain Road, the four divisions in reverse order in column one behind the other,  Marcognet’s Division, which the 21eme formed a part, was in the second line. At 11.30am, the Army Corps held its left on the farm of La Belle Alliance, the right towards the farm of La Haye. At midday, the 62 cannon covering the corps opened fire. Count d’Erlon formed his divisions en echelon. Each division formed of a column of battalions, deployed in “close” column at a distance of 5 paces from each other. Being formed, the column advanced through the muddy and high crop covered fields. As they reached the plateau, the artillery ceased fire.

D’Erlon had the charge beaten, the soldiers of Marcognet’s Division threw themselves on the battalions of Pack’s English brigade, and Best’s Hanoverian brigade. The latter, partly lying down in the wheat rose up and opened a heavy fire on to the battalions of the division, as they crossed the hedges which lined the Ohain road, they moved down the reverse of the ridge. At this moment a charge of 1,200 of Ponsonby’s English Dragoons brought disorder into the ranks of the division which didn’t have time to form square because of the faulty dispositions of the division. The soldiers of the Division, sabred by the English Dragoons, where obliged to quickly retreat into the valley. The Division was reformed and returned to the plateau, where it remained without being able to advance or retreat until the end of the day. It was subsequently attacked by the infantry of the Prince of Saxe Weimar as it descended the slope; disorder began to take over the Division, the men left the ranks and arrived in disorder towards La Belle Alliance, under canister from 32 pieces placed in a battery on the plateau. Steinmetz’s cavalry appeared,  Napoleon hade formed four squares from the battalions of the Guard in a supreme effort to save the situation. He tried in vain to rally Marcognet’s and Durette’s men; each moment more and more men were fleeing; until d’Erlon’s Corps did not have any formed units, his entire artillery was taken. The men fled in the direction of Marchiennes, where the debris of the army recrossed the Sambre. The Battle of Waterloo was lost.

The Colonel of the 21eme was taken prisoner. Amongst the wounded of the Regiment, were Chef de Bataillon Debar, captains Bourgogne and Thory, lieutenants Soufflet, Tercine, and Benoit, and eagle bearer Fleury.

After Waterloo, the army retreated towards Paris, command was given to Marshal Davout. On the 24^th, the debris of the 21eme rallied at Laon, the roll-call made at six in the evening stated that there were 12 officers and 158 men present. On the 28^th the Regiment arrived at Paris with an effective strength of 18 officers and 206 men, taking position at Belleville, the headquarters of General Count d’Erlon, commander of the right wing of the army united at Paris.

Wellington and Blucher deployed 100,000 before the entrenched camp at Paris. Napoleon who had abdicated on the 22^nd June, left the capital on the 29^th, the allies entered Paris on the 6^th July. Louis XVIII arrived two days later.

The Army of Waterloo, with a strength of 120,000 men and 500 cannon, withdrew behind the Loire,  it gave its submission to the King on the 13^th July, but soon the Allies demanded the dissolution of all those proud regiments which they still feared,  they were to be broken up and disarmed.

Marshal Macdonald was ordered to Bourgesto oversee the dissolution as prescribed by the Order of the 16^th July. On the 1^st August, all the units were broken up and each without disorder were put on the road.

The Colonel of the 21eme, Lechevalier Ledoux, remained at President of the Council of Administration up to the disbandment of the Regiment which took place on the 10^th March 1816. Captaine d’habillement Venziac deposited the Regimental archives at the same time.