During the War of the Third Coalition the French Army of Italy under Marshal André Masséna faced Archduke Charles, Duke of Teschen's Austrian army. Lacour led a brigade in Julien Augustin Joseph Mermet's cavalry division consisting of four squadrons each of the 24th and 30th Dragoon Regiments. At the Battle of Caldiero on 29–31 October 1805, Lacour's brigade was attached to Guillaume Philibert Duhesme's 4th Division. On the 30th, Duhesme's division deployed on the right with Lacour's dragoons guarding the army's extreme right flank. The plan allowed for Jean-Antoine Verdier's division to cross the Adige River and extend Duhesme's right. Masséna assigned Duhesme the task of capturing the town of Caldiero. In the event, Verdier's attack miscarried and Duhesme captured Caldiero without help. However, the day ended in stalemate after bitter fighting. Every unit was committed to the fray except Mermet's other brigade which was made up of cuirassiers. On the 31st, Verdier's relatively fresh division attacked but was stopped. Charles ordered his army to retreat the next day.