The Second Battle of Porto, also known as the Battle of the Douro or the Crossing of the Douro, took place on 12 May 1809. General Arthur Wellesley's Anglo-Portuguese Army defeated Marshal Soult's French troops and took back the city of Porto. After taking command of the British troops in Portugal on 22 April, Wellesley immediately advanced on Porto and made a surprise crossing of the Douro River, approaching Porto where its defences were weak. Soult's late attempts to muster a defence were in vain. The French quickly abandoned the city in a disorderly retreat.

Louis Henri Loison briefly joined the French Army in 1787 and after the French Revolution became a junior officer. Blessed with military talent and courage, he rapidly rose to general officer rank during the French Revolutionary Wars. He got into difficulties because of his fondness for plundering. In late 1795 he helped Napoleon Bonaparte crush a revolt against the government. After a hiatus, he returned in 1799 to fight in Switzerland where he earned another promotion. In 1800 he commanded a division under Napoleon in the Marengo campaign.