Grandjean led a brigade in Claude Perrin Victor's division at the Battle of Trebbia. Under his command were the 5th, 39th and 92nd Line Infantry Demi-brigades, a total of 3,765 foot soldiers. The army commander Étienne MacDonald was feverish from a wound and was unable to coordinate his divisions effectively. On 17 June the leading divisions of Victor, Jean-Baptiste Dominique Rusca, Jean Henri Dombrowski and Jean-Baptiste Salme attacked without orders and were repulsed at the Tidone River. The next day the French were able to hold off the Austro-Russian attacks. On the 19 June, MacDonald ordered a general assault which was beaten back with heavy losses. Victor exploited a gap in the enemy lines but he was compelled to withdraw after the defeat of the other divisions. French generals Victor, Rusca, Dombrowski, Salme, Grandjean, Jean-Baptiste Olivier and Alexis Aimé Pierre Cambray were wounded, the last fatally.