A skilled tactician, he was one of the veteran generals who made the Napoleonic armies such a formidable foe to the other European powers.

[1] During the War of the Second Coalition, Grenier led a division at the battles of Verona, Magnano, and Cassano d'Adda in the spring of 1799.

[1] While leading 8,000 men of his division, he defeated Friedrich Heinrich von Gottesheim and 5,000 Austrians at Fossano on 16 September 1799.

On 1 December, he supervised the divisions of Michel Ney and Jean Hardÿ in a successful rearguard action in the Battle of Ampfing.

"[7] Two days later, he commanded the three divisions of Ney, Louis Bastoul (for the wounded Hardÿ), and Claude Legrand in Moreau's decisive victory at the Battle of Hohenlinden.

At the start of the 1809 campaign, Grenier commanded an infantry division in Eugène de Beauharnais' Army of Italy.

[12] He intercepted and crushed Franz Jellacic's Austrian division in the Battle of Sankt Michael on 25 May, where he demonstrated his superior generalship.

"[16]The spring of 1813 found Grenier serving under Eugène in Saxony as Napoleon assembled a new army to replace the one destroyed in Russia.

[17] After the summer armistice, Eugène and Grenier returned to Italy to prepare the Italian army for hostilities with Austria.

When Hiller established a bridgehead across the Drava River at Feistritz an der Drau, Grenier attacked and defeated the Austrians on 6 September.

[19] The Franco-Italian strategic situation soon deteriorated and Eugène slowly withdrew to the Adige River in October and November.

On 31 October, Grenier won a smart victory over Christoph Ludwig von Eckhardt's column at Bassano del Grappa, sending the Austrians fleeing into the mountains.