In the war of 1806 against Napoleon, Barclay took a distinguished part in the Battle of Pultusk (December 1806) and was wounded at the Battle of Eylau (7 February 1807), where his conduct won him promotion to the rank of lieutenant general. After a period of convalescence, Barclay returned to the army and in 1808 commanded operations against the Swedes during the Finnish War. In 1809, he successfully marched over the frozen Gulf of Bothnia, which allowed him to surprise the enemy and seize Umeå in Sweden. For this exploit, immortalized by the Russian poet Baratynsky, he was made full general and Governor-General of Finland. A year later, he became Minister of War, retaining the post until 1813.