[2][4] Lannes received little education, but his great strength and proficiency in many sports caused him in 1792 to be elected sergeant-major of the battalion of volunteers of Gers, which he had joined upon the outbreak of war between France and Spain.

During his time in the Pyrenees, Lannes was given some important tasks by General Jacques François Dugommier and recommended for promotion by future Marshal Louis-Nicolas Davout.

[9] When he and a small reconnaissance party ran into 300 Papal cavalry, he averted danger by astutely ordering the men to return to base, convincing them not to attack.

[7][9] He was chosen by Bonaparte to accompany him to Egypt as commander in one of General Jean-Baptiste Kléber's brigades,[10] in which capacity he greatly distinguished himself, especially during the retreat from Syria.

[6][12] General Joachim Murat and Chef de brigade Jean-Baptiste Bessières schemed to have Lannes removed over a budget deficit,[13] but Augereau bailed him out.

[6] In 1807, Napoleon recreated the Duchy of Siewierz (Sievers), granting it to Lannes after Prussia was forced to cede all her acquisitions from the second and third partitions of Poland.

After this, Lannes was to be tested as a commander-in-chief, for Napoleon sent him to Spain in 1808 and gave him a detached wing of the army to command, with which he won a crushing victory over General Francisco Castaños at Tudela on 22 November.

With his corps, he led the French Army across the Danube River and bore the brunt, with Marshal André Masséna, at the Battle of Aspern-Essling.

He was continually employed in tasks requiring the utmost resolution and daring, and more especially when the emperor's combinations depended upon the vigour and self-sacrifice of a detachment or fraction of the army.

It was thus with Lannes at Friedland and at Aspern as it was with Davout at Austerlitz and Auerstedt, and Napoleon's estimate of his subordinates' capacities can almost exactly be judged by the frequency with which he used them to prepare the way for his own shattering blow.

Dependable generals with the usual military virtue, or careful and exact troop leaders like Jean-de-Dieu Soult and Étienne Macdonald, were kept under Napoleon's own hand for the final assault which he himself launched; the long hours of preparatory fighting against odds of two to one, which alone made the final blow possible, he entrusted only to men of extraordinary courage and high capacity for command.