Napoleon appointed Boussart a Baron of the Empire on 19 March 1808 and posted the much-wounded veteran to Spain. In 1808 he was with Pierre Dupont de l'Étang's corps in Andalusia. He led a cavalry brigade that consisted of the 6th Provisional Dragoon Regiment and numbering 620 troopers. On 17 July his command was detached with Dominique Honoré Antoine Vedel's division to march to Bailén. However, Vedel marched beyond the town in a mistaken belief that the Spanish had seized the passes to the north. In fact, Theodor von Reding's 17,000-man Spanish corps arrived at Bailén on the 18th and occupied the town without opposition. This placed Dupont's 11,000 men squarely between Reding and Francisco Javier Castaños's corps. In the Battle of Bailén, Dupont failed to break out of trap and asked for terms. While the negotiations were going on, Vedel's division returned. Boussart's dragoons attacked and captured about 1,000 Spanish troops. When a Spanish delegation warned Vedel that there was a truce in force, he ordered his soldiers to cease fire. When Dupont ordered him to release the prisoners taken during the parley, Vedel did so. As Dupont surrendered, Vedel began retreating north in accordance with secret orders from his superior. Later, when the Spaniards threatened to massacre Dupont's men unless Vedel was included in the capitulation, Dupont sent an order to Vedel to return to Bailén and surrender. Amazingly, Vedel followed the order even though his division was not surrounded. This more than doubled the total of unwounded prisoners taken from 8,242 to 17,635. While Dupont and the French generals were returned to France, the enlisted men spent the rest of the war in prison hulks or on the bleak island of Cabrera and half of them died of disease or starvation.