During the years preceding the War of 1812, Dickson and other British and Canadian traders were angered by American encroachments into the area where they traded, in which they had previously enjoyed a monopoly. As war appeared imminent, Dickson began recruiting warriors from among the tribes with which he traded, and gathered them at the British military outpost at Fort St. Joseph. When news of the outbreak of war arrived, Dickson and his followers joined the garrison of Fort St. Joseph, led by Captain Charles Roberts, in an attack on the American fort at Mackinac Island. They successfully captured the island from its unwary American garrison. He subsequently led his followers south to join the British army at Amherstburg, where they took part in the Siege of Detroit, which caused the surrender of an American army. In the autumn following these victories, Dickson travelled to Montreal, where he was appointed to the Indian Department as Agent and Superintendent for the Western Indian tribes.