In his fifteenth year he entered the army as a sub-lieutenant in an infantry regiment, by the officers of which he was noticed for the uncommon gravity of his disposition. In the wars of the Revolution he attracted still more attention for a valour combined with discretion, for the promptitude of his measures, and for his almost unvaried success. His promotion was in consequence rapid. In 1796, he served under Moreau, as general of division; he commanded the left wing at the battle of Rastadt; and whenever he was entrusted with any separate or detached enterprise, the wisdom and decision of his combinations pointed him out as one of the most scientific officers in the French armies.

In the Egyptian expedition, Dessaix contributed powerfully to the triumphs of Buonaparte. He defeated the Arabs of Yambo, and the Mamelukes of Murad-Bey, and thereby reduced the whole of Upper Egypt; where, however, a general of less prudence would have fainted under the burden of victory. Provisions were scarce; the heat was intense; the country wholly unknown; and the enemy, though defeated, never failed to hover on his flanks, to intercept stragglers, and, when resisted, to retire with the speed of the wind. These circumstances were exceedingly discouraging to the army, even to the bravest; but such was his popularity, and the well-known wisdom of his measures, that he was enabled to preserve subordination.

When Buonaparte left Egypt, he expressed his wish that Dessaix. should follow him as soon as an opportunity occurred. The Convention of El Arish enabled him to do so. On his arrival in France, he learned the First Consul had passed the Great St. Bernard, and was advancing into Italy: "He will leave us nothing to do!" was his impatient observation as he hastened to join his friend. He travelled day and night until he reached head-quarters. The Consul embraced him with great affection, and immediately despatched him with a division to reconnoitre the road to Genoa. Scarcely, however, had he departed, before Melas, the Austrian general, advanced against the less numerous forces of Buonaparte. Dessaix was recalled, but he did not arrive until the French were retreating. He rode up to his friend, consternation visibly painted on his brow, and said: "I think the battle is lost!" "I think it is won!" answered Napoleon; "Push on, I will rally the line behind you." Though Dessaix had travelled ten leagues without halting, he instantly formed his columns, and charged the Austrians with such impetuosity that they gave way in every direction. Just at that moment, a ball struck him in the head, and he fell, to rise no more. "Tell the First Consul," were his last words, "that I die with regret, since I have not lived long enough for glory!"

The death of this beloved chief roused the French to vengeance; the enemy was pursued with fury. When Buonaparte heard of the catastrophe, he was in the heat of the pursuit: "Alas!" said he, "I have no leisure to weep;" and there is no doubt he felt the loss severely. He caused the corpse to be embalmed, and conveyed to the hospice of St. Bernard, where a monument was raised to the memory of the fallen hero.

Dessaix was of a mild as well as of a decided character; and his integrity was such that by the inhabitants of Upper Egypt, he was called the Just Sultan. "Of all the generals I ever had," said Buonaparte at St. Helena, "Dessaix and Kleber had the greatest talents: their loss was irreparable to France." The former he considered as second only to himself in military merit. "Dessaix thought only of glory," said Napoleon; "he lived on glory. Luxury he despised, and even comfort. He preferred sleeping under a gun in the open air, with his cloak wrapt about him, to the softest couch. Money he totally disregarded."