An American army commanded by William Hull invaded Upper Canada on July 12, arriving at Sandwich (Windsor, Ontario) after crossing the Detroit River.^[5] His forces were chiefly composed of untrained and ill-disciplined militiamen.^[6] Hull issued a proclamation ordering all British subjects to surrender, or "the horrors, and calamities of war will stalk before you".^[7] The proclamation said that Hull wanted to free them from the "tyranny" of Great Britain, giving them the liberty, security, and wealth that his own country enjoyed—unless they preferred "war, slavery and destruction".^[8] He also threatened to kill any British soldier caught fighting alongside indigenous fighters.^[7] Hull's proclamation only helped to stiffen resistance to the American attacks as he lacked artillery and supplies. Hull also had to fight just to maintain his own lines of communication.^[9]

Hull withdrew to the American side of the river on 7 August 1812 after receiving news of a Shawnee ambush on Major Thomas Van Horne's 200 men, who had been sent to support the American supply convoy. Hull had also faced a lack of support from his officers and fear among his troops of a possible massacre by unfriendly indigenous forces. A group of 600 troops led by Lieutenant Colonel James Miller remained in Canada, attempting to supply the American position in the Sandwich area, with little success.^[10]