A keen sportsman, in peace he spent all his leisure in the chase. Hard exercise and feats of physical endurance were his delight. Fatigue he never knew, and on one occasion he was said to have spent thirteen days and nights in the saddle. To strangers and officials he was silent and stern, but his aloofness of manner hid a warm heart and a natural sincerity, and many a poor officer or returned émigré received secret help from his purse. Though naturally of a strong character, his affection and respect for his great commander became the dominating note in his career; in fact, it might almost be said that, in later years, his personality became merged to such an extent in that of Napoleon that he was unable to see the actions of the Emperor in their proper perspective. From their first meeting Bonaparte correctly guessed the impression he had made on his new staff officer, and aimed at increasing his influence over him. Meanwhile he was delighted with him, he wrote to the Directory, "Berthier has talents, activity, courage, character—all in his favour." Berthier on his side was well satisfied; as he said to a friend who asked him how he could serve a man with such a temper, "Remember that one day it will be a fine thing to be second to Bonaparte." So the two worked admirably together.

Bonaparte kept in his own hands the movement of troops, the direction of skirmishes and battles, commissariat, discipline, and all communications from the Government. Berthier had a free hand in the organisation and maintenance of the general staff, the headquarter staff, and the transmission of orders, subject to inspection by Bonaparte; he also had to throw into written form all verbal orders, and he alone was responsible for their promulgation and execution. It was his ability to work out in detail and to reduce into clear, lucid orders the slightest hint of his commander which, as Napoleon said later, "was the great merit of Berthier, and was of inestimable importance to me. No other could possibly have replaced him." Thanks to Berthier's admirable system, Bonaparte was kept in touch with every part of his command. One of the first principles laid down in the staff regulations was, "That it was vital to the good of the service that the correspondence of the army should be exceedingly swift and regular, that nothing should be neglected which might contribute to this end." To ensure regularity of communication, divisional commanders and officers detached in command of small columns were ordered to report at least twice a day to headquarters. With each division, in addition to the divisional staff, there were officers detached from the headquarters staff. All important despatches had to be sent in duplicate; in times of great danger commanding officers had to send as many as eight different orderly officers each with a copy of despatches.

But it was not only as an organiser and transmitter of orders that Berthier proved his usefulness to his chief. At Lodi he showed his personal courage and bravery among the band of heroes who forced the bridge, and Bonaparte paid him a fine tribute when he wrote in his despatches, "If I were bound to mention all the soldiers who distinguished themselves on that wonderful day, I should be obliged to mention all the carabiniers and grenadiers of the advance guard, and nearly all the officers of the staff; but I must not forget the courageous Berthier, who on that day played the part of gunner, trooper, and grenadier." At Rivoli, in addition to his staff duties, Berthier commanded the centre of the army, and fought with a stubbornness beyond all praise. By the end of the campaign of 1796 he had proved that he was as great a chief of the staff as Bonaparte was a great commander. Doubtless it is true that before the commencement of a campaign an army possesses in itself the causes of its future victory or defeat, and the Army of Italy, with its masses of enthusiastic veterans and the directing genius of Bonaparte, was bound to defeat the Austrians with their listless men and incompetent old generals; but, without the zeal, activity, and devotion which Berthier transfused through the whole of the general staff, success could not have been so sudden or so complete.

After Leoben the conqueror of Italy employed his trusty friend on numerous diplomatic missions in connection with the annexation of Corfu and the government of the Cisalpine republic. Meanwhile he was in close communication with him in regard to the proposed descent on England and the possible expedition to the East. To Berthier, if to any one, Bonaparte entrusted his secret designs, for he knew that he could do so in safety. Accordingly, in 1798, finding an invasion of England impossible at the moment, he persuaded the Directory to send Berthier to Italy as commander-in-chief, his object being to place him in a position to gather funds for the Egyptian expedition. From Italy Berthier sent his former commander the most minute description of everything of importance, but he found the task difficult and uncongenial, and prayed him "to recall me promptly. I much prefer being your aide-de-camp to being commander-in-chief here." Still he carried out his orders and marched on Rome, to place the eight million francs' worth of diamonds wrung from the Pope to the credit of the army. From Rome he returned with coffers well filled for the Egyptian expedition, but leaving behind him an army half-mutinous for want of pay; his blind devotion to Bonaparte hid this incongruity from his eyes.

As in Italy in 1795 so in Egypt, Berthier was Bonaparte's right-hand man, methodical, indefatigable, and trustworthy. But even his iron frame could scarcely withstand the strain of three years' continuous active service, the incessant office work day and night, and the trials of an unaccustomed climate. After the battle of the Pyramids he fell sick, and before the Syrian expedition, applied to return to France. Unkind friends hinted that he longed for his mistress, Madame Visconti, but Bonaparte, knowing that it was not this but sheer overstrain which had caused his breakdown in health, gave him the desired leave and made all arrangements for his journey home. However, at the moment of departure Berthier's love for his chief overcame his longing for rest, and, in spite of ill-health, he withdrew his resignation and set out with the army for Syria. As ever, he found plenty of work, for even in the face of the ill-success of the expedition, Bonaparte determined to administer Egypt as if the French occupation was to be for ever permanent; and Berthier, in addition to his ordinary work, was ordered to edit a carefully executed map from the complete survey which was being made of the country.

It was to Berthier that Bonaparte first divulged his intention of leaving Egypt and returning to France, and his determination to upset the Directory. Liberal by nature, but essentially a man of method and a disciplinarian, the chief of the staff was quite in accord with his commander's ideas on the regeneration of France, and loyally supported him during the coup d'état of the 18th Brumaire. Thereafter the First Consul appointed his friend Minister of War, a position that gave full scope to his talents. All the administrative services had at once to be reorganised, the frontier fortresses garrisoned and placed in a state of defence, and the army covering the frontiers supplied with food, pay, equipment, and reinforcements, while the formation of the secret Army of Reserve was a task which alone would have occupied all the attention of an ordinary man; in fact, the safety of France hung on this army. Consequently, since, by the constitution, the First Consul was unable himself to take command in the field, in April, 1800, he transferred Berthier from the War Office to the head of this most important force. It is not generally known that the idea of the passage of the Alps by the St. Bernard Pass actually originated with Berthier, and had first been projected by him as early as 1795. So it was at the execution of what was really his own idea that for two months Berthier slaved. At times even his stout heart quailed, as when he wrote to the First Consul, "It is my duty to complain of the position of this army on which you have justly spent so much interest, and which is paralysed because it can only rely on its bayonets, on account of the lack of ammunition and means to transport the artillery." Incessant work and toil were at last rewarded; but when the Army of the Reserve debouched on the Austrian lines of communication, the First Consul appeared in person, and, though nominally in command, Berthier once again resumed his position of chief of the staff. Without a murmur he allowed Bonaparte to reap all the glory of Marengo, for he knew that without the First Consul, however excellent his own dispositions were, they would have been lacking in the driving power which alone teaches men how to seize on victory. After Marengo, Berthier was despatched as Ambassador Extraordinary to Madrid, "to exhort Spain by every possible means to declare war on Portugal, the ally of England." The result of this mission was eminently successful; a special treaty was drawn up and Spain sold Louisiana to France. By October the ambassador was once again back in Paris at his old post of Minister of War—a post which he held continuously during peace and war till August, 1807. The position was no light one, for even during the short years of peace it involved the supervision of the expedition to San Domingo, the defence of Italy, the reorganisation of the army, and the re-armament of the artillery, in addition to the ordinary routine of official work. Moreover, the foundations of the Consulate being based on the army, it was essential that the army should be efficient and content, and consequently the French soldier of that day was not, as in other countries, neglected in peace time. The officers in command of the troops were constantly reminded by the War Minister that "the French soldier is a citizen placed under military law"—not an outcast or serf, whose well-being and comfort concern no one.

On the establishment of the Empire Berthier, like many another, received the reward for his faithfulness to Napoleon. Honours were showered upon him. The first to receive the Marshal's bâton, he was in succession created senator by right as a dignitary of the Empire, grand officer of the palace and grand huntsman to the crown, while at the coronation he carried the imperial globe. But though the Emperor thus honoured, and treated him as his most trustworthy confidant, the cares of state to some extent withdrew Napoleon from close intimacy with his old companion. At the same time the Marshal was insensibly separated from his former comrades-in-arms by his high rank and employment, which, while it tended to make him more the servant than the friend of the Emperor, also caused him to be regarded as a superior to be obeyed by those who were formerly his equals. At all times a strict disciplinarian, and one who never passed over a breach of orders, the Marshal, as voicing the commands of the Emperor, gradually began to assume a stern attitude to all subordinates, and spared neither princes or marshals, when he considered that the good of the service required that they should be reprimanded and shown their duty. So strong was the sense of subordination in the army and the desire to stand well with Napoleon, that even the fiery Murat paid attention to orders and reprimands signed by Berthier in the name of the Emperor.

Meanwhile the work of the War Minister increased day by day. The organisation and supervision of the Army of the Ocean added considerably to his work, which was much interfered with by visits of inspection in company with the Emperor, or far-distant expeditions to the frontiers and to Italy for the coronation at Milan.

On August 3rd, 1805, the Emperor created the Marshal major-general and chief of the staff to the Army of the Ocean, and himself assumed command of the Army and held a grand review of one hundred thousand men. Everybody thought that the moment for the invasion of England had arrived. Berthier, and perhaps Talleyrand, alone knew that Austria, not England, was the immediate quarry, and all through August the major-general was busy working out the routes for the concentration of the various corps in the valley of the Danube; whilst at the same time as War Minister he was responsible for the supervision of all the troops left in France and in garrison in Italy, Belgium, Holland, and Hanover. Consequently he had to divide his staff into two sections, one of which he took with him into the field, the other remaining in Paris under an assistant who was capable of managing the ordinary routine, but who had to forward all difficult problems to the War Minister in the field. Even during the drive to the frontier there was no abatement of the strain; during the journey the Emperor would give orders which had to be expanded and written out in the short stoppages for food and rest. By day the major-general travelled in the Emperor's carriage; at night he always slept under the same roof with him, to be ready at any moment, in full uniform, to receive his commands and expand and dictate them to his clerks. Everyone knew when the major-general was worried, for he had a habit of biting his nails when making a decision or trying to solve a problem, but otherwise he never showed any sign of feeling, and whether tired or troubled by the Emperor's occasional outbursts of temper, he went on with his work with the methodical precision of an automaton. To belong to the general staff when Berthier was major-general was no bed of roses, no place for gilded youth, for with Napoleon commanding and Berthier directing, if there was often fighting there was plenty of writing; if there was galloping on horseback by day, to make up for it by night there were hours of steady copying of orders and no chance of laying down the pen until all business was finished. Thanks to this excellent staff work, Napoleon's ambitious plans were faithfully accomplished, the Austrians were completely taken in by the demonstration in the Black Forest, the French columns stepped astride of their communications on the Danube, and Mack was forced to surrender at Ulm. But Ulm was only the commencement of the campaign, and even after Austerlitz Napoleon pursued the enemy with grim resolution. This was one of the secrets of his success, for, as Berthier wrote to Soult, "The Emperor's opinion is that in war nothing is really achieved as long as there remains something to achieve; a victory is not complete as long as greater success can still be gained."