In 1805, Loison led a division in Napoleon's Grande Armée during the Ulm Campaign and served in the War of the Fourth Coalition in 1806 and 1807. He saw much action in the Peninsular War including all three invasions of Portugal, though not always with distinction. In Portugal he earned a bad name for his harshness and the inhabitants called him "Maneta" or One-Hand. For a brief period, he commanded Michel Ney's famous VI Corps. During the French invasion of Russia he assembled a reserve division, which was later destroyed by the extreme cold weather. He served in the War of the Sixth Coalition before a harsh rebuke from the emperor effectively ended his military career. Loison is one of the names inscribed under the Arc de Triomphe.