In 1807 Margaron was assigned to the 1st Corps of Observation of the Gironde under Jean-Andoche Junot. Margaron and Antoine Maurin led brigades in the 1,754-man cavalry division under François Étienne de Kellermann. The mounted troops included one squadron each from the 1st, 3rd, 4th, 5th, 9th and 15th Dragoon and 26th Chasseurs à Cheval Regiments. All squadrons counted between 236 and 262 troopers. With Spain's permission, Junot's force crossed the Bidasoa River on 18 October 1807 and was in Salamanca by 12 November, prepared to invade Portugal. Little did the Spanish know that Napoleon soon planned to overthrow their kingdom as well. Soon after Junot's corps launched the 1807 invasion of Portugal, logistical arrangements broke down and half the army's horses died. Nevertheless, on 30 November 1807, a 1,500-man French vanguard occupied Lisbon, having faced no military opposition. Then cavalrymen then remounted themselves with confiscated horses.