In 1800, he served under Jean Moreau, fighting at the Battle of Höchstädt on 18 and 19 June 1800. After the summer truce expired in November, Moreau placed him in charge of the three-division Left Wing. On 1 December, he supervised the divisions of Michel Ney and Jean Hardÿ in a successful rearguard action in the Battle of Ampfing. One historian writes, "Grenier possessed an unmatched knowledge of complex tactical evolutions." 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.