The British commander in Upper Canada had…
July 1812 CE
In response to the U.S. declaration of war, Isaac Brock had issued a proclamation alerting the citizenry in Upper Canada of the state of war and urging all military personnel "to be vigilant in the discharge of their duty" to prevent communication with the enemy and to arrest anyone suspected of helping the Americans.
He had also issued orders to the commander of the British post at Fort St. Joseph to initiate offensive operations against U.S. forces in northern Michigan, who it turns out, were not yet aware of their own government's declaration of war.
The resulting Siege of Fort Mackinac on July 17 had been the first major land engagement of the war, and ended in an easy British victory.