Rejecting the brilliant match proposed by the First Consul, he chose as his bride Mademoiselle Lapezrière, a young lady of a royalist family. The couple were married by a nonjuring priest, and, far from incurring displeasure, were greatly complimented, for Bonaparte already desired the Concordat with the Pope, and saw in the bride a useful supporter of his scheme. Madame Bessières was a great social success: a favourite of Napoleon and a close friend and confidant of Josephine; everywhere she was welcomed for her beauty, her force of character, and the charm of her manner.

During the year of peace and the preparation for the invasion of England, Bessières accompanied the First Consul on all his numerous expeditions. To his credit be it said, he protested loudly against the ill-judged execution of the Duc d'Enghien. When the First Consul became Emperor he enrolled his friend among his new Marshals, not for his military genius, but as a reward for his fidelity, for none knew better than Napoleon how lacking the new Marshal was in many of the requisites of a great commander.

In 1805 the cavalry of the Guard formed part of the Grand Army, and their commander, by his able backing of Murat, had his share in helping to win the battle of Austerlitz. During the interval between the Austrian and the Prussian campaigns the Marshal was busily occupied in Paris in reorganising and expanding the Guard, and, as usual, was in close touch with the Emperor. In the Prussian campaign Bessières had his first taste of an independent command, and gained great credit for his masterly manœuvring in Poland, where with a weak force he kept the enemy in complete ignorance of the movements of the French, and covered the conjunction of the various corps of the army.

After the peace of Tilsit he was entrusted with the delicate mission of negotiating a marriage between Princess Charlotte of Würtemburg and Prince Jerome, the new King of Westphalia. Hardly had he returned to Paris when he was hurried off again on active service, this time to Spain. It was just a week before the disaster of Baylen that Marshal Bessières was confronted with a most serious problem. The Spanish levies from Old Castile, under Cuesta, had effected a junction with the levies of Galicia, under Blake, and were threatening to overwhelm the weak force of ten thousand men with which the Marshal was attempting to put down the guerilla warfare in the northern provinces. Bessières had not been the great Emperor's confidant for nothing, and he at once saw that, unless he took the initiative, his force was doomed, for the enemy were in overwhelming strength, and every day added to their numbers. He knew well how ill-disciplined their forces were, and he determined to try the effect of a surprise. Everything fell out as he wished. On July 14th he found the Spanish armies in position outside Medina del Rio Seco, some few miles east of Valladolid. The Spaniards, not knowing whether the French were advancing from the direction of Valladolid or Burgos, had placed the army of Blake on the Valladolid road, and that of Cuesta on the Burgos road. Accordingly the Marshal was able to surprise and defeat Blake, and then to turn and inflict a similar defeat on Cuesta. So far his dispositions had been excellent, but, as General Foy said, "He could organise victory, but he could not profit by it," for he was paralysed by the extent of the guerilla warfare with which he was faced, and after a short but bloody pursuit he called off his troops. Still, he had accomplished much; for the time he had dispersed all organised resistance in the northern provinces, and had opened the road to Madrid for King Joseph.

But Baylen and Vimeiro proved that the war in the Iberian Peninsula was still only in its first stage. Joseph had hastily to evacuate Madrid, and, in spite of having twelve thousand French troops under his command, Bessières could effect nothing. The Spanish armies of Cuesta and Blake once again took shape; and, like the other French generals, the Marshal had to fall back on the line of the Ebro. Such was the situation in October when the Emperor himself appeared on the scene. The situation changed like magic at the touch of a master hand. The French troops, strung out in a great semicircle on the Ebro, were quickly concentrated. Blake and Cuesta were each defeated by an overwhelming combination of the different French armies. Meanwhile, the Emperor, recognising the limitations of his faithful friend, superseded him by Soult, but gave him the command of the Guard and of the reserve cavalry, under his own immediate supervision, and took him back to France when he gave up the pursuit of the English.

Napoleon desired to take the Guard with him on the Austrian campaign, and, as several regiments were still in Spain, others had to be enrolled to take their places. These regiments were entirely organised by Bessières, and formed the nucleus of what was later called the Young Guard. The Marshal's duty during the Austrian campaign of 1809 was the same as in Spain: the command of the Guard and of the reserve cavalry. During the famous Five Days' Fighting he proved again that no troops in Europe could resist the charges of the heavy cavalry of the Guard, and that he himself had almost as great a command of the technique of cavalry tactics as his famous friend and instructor, the King of Naples. At Aspern and Essling the cavalry of the Guard and the reserve cavalry covered themselves with glory by their dashing charges. Again and again, with cries of "Vive l'Empereur," the glittering masses of cuirassiers attempted to break down the stern handful of indomitable Hungarians who guarded the Austrian batteries. When the bridges were broken, and the retreat to the island of Lobau was the only hope for the army, Bessières, with the remains of cavalry, so severely punished the enemy that the retirement was effected in safety. At Wagram, when all seemed lost, Napoleon called on his old comrade to sacrifice himself with his cavalry. As the cuirassiers of the Guard trotted past to debouch on their heroic mission, the Emperor, waving his sword, cried out, "No sabring. Give point, give point!" The needed time was gained, and the gallant Marshal was wounded. But at the end of the day, when the troopers, after their great effort, could no longer face the unbroken lines of slowly retreating Austrians, Napoleon, chagrined at his failure, met his cavalry and their commander with reproach: "Was ever anything seen like this? neither prisoners nor guns! This day will be attended with no result."

The Emperor's ill-humour was only temporary. When his most trusted lieutenants were grumbling and longing for peace in which to enjoy the spoil they had collected in war, when Bernadotte and Fouché were openly intriguing against him, Napoleon could ill afford to disregard his most faithful friend. Accordingly, immediately after Wagram he despatched the newly created Duke of Istria to Belgium to take over the command of the French troops who were opposing the ill-fated English expedition to the isle of Walcheren. When the Marshal returned from Belgium to Paris he found that the Emperor had made all arrangements for the divorce of Josephine and for his second marriage. Bessières was placed in a very awkward position. Prince Eugène was his greatest friend. Josephine had always been most kind to him and the Duchess, but he could not help them in any way, and, to make matters worse, the Emperor insisted on coming and staying with him at his country house at Grignon.

Meanwhile the war in Spain was spoiling many great reputations. Reinforcements were urgently required, so the Emperor decided to give his Young Guard their baptism of fire in Spain. Accordingly, at the commencement of 1811 he despatched them with Bessières, their commander, to operate on the northern lines of communication. The ill-success of the French was palpably due to two causes. There was no commander-in-chief on the spot—the Emperor was in Paris—and there was no other Marshal whom all the others would obey. Secondly, there was a great want of concentration; as Bessières wrote to Berthier: "All the world is aware of the vicious system of our operations, everyone sees that we are too much scattered. We occupy too wide an extent of country: we exhaust our resources without profit and without necessity: we cling to dreams. We should concentrate our forces; retain certain points d'appui for the protection of our magazines and hospitals, and regard two-thirds of Spain as a vast battlefield, which a single victory may either secure or wrest from us." Unfortunately the Marshal was human, like his comrades, and instead of loyally backing up Masséna, he came to an open rupture with him on the question of supplies, and by his inaction at Fuentes d'Onoro he caused the French to lose that battle. Though he made good his excuses before Napoleon, and secured the disgrace of the Prince of Essling, in the opinion of the Duke of Wellington it was Bessières's refusal to lend Masséna assistance which was entirely responsible for the French defeat. Moreover, sound as were his views on the method of conducting war, he had not the personality to impress them on others or the application to put them into practice, and his whole time was occupied in attempting to make head against the guerilla warfare. His methods were rough and barbarous, and reacted against the French, for he avenged the ill deeds of the guerillas on their families and women folk, and visited with military execution any village which failed to meet his onerous requisitions. So the Spaniards retaliated with revenge, the weapon of the weak, that "wild kind of justice." The Marshal's blunders were cut short by his recall to Paris at the beginning of 1812 to reorganise the Guard prior to the Russian campaign.

The Duke of Istria accompanied the Emperor to the front. His individual share was restricted by the fact that the King of Naples was with the army. But during the retreat he led the van and did yeoman service in restoring order among the disheartened troops.

Early in 1813 he was recalled from Ebling to reorganise the Guard and the reserve cavalry. The task tried to the utmost the Marshal's great administrative capacity, for not only was there the question of men and equipment, but above all he was confronted with the difficulty of providing remounts. In spite of all his efforts it was impossible to find anything like enough horses for the cavalry, for the guns had to be supplied first.