The battalion advanced north into Portugal with the main force of the army, whose first objective was the recapture of Oporto. In the Second Battle of Porto, the light company of the Buffs (Royal East Kent Regiment) crossed the river by boat and captured the seminary, a strategically located building, along with troops from Rowland Hill, 1st Viscount Hill's Brigade. while the main force entered the town from a different direction. The defenders were surprised while withdrawing, and took heavy casualties. The force pursued the French army into Spain, with the 83rd's brigade engaging and routing the French rear-guard at Salamonde on the 16th before breaking off the pursuit. The force then was garrisoned along the Tagus River, where it suffered heavily from illness spreading among the men.