The United States declared war on Britain on 18 June 1812. Officially, the American declaration was a response to the Royal Navy's repeated violations of the "freedom of the seas." The occupation of Canada would also stop the flow of arms and ammunition to the Shawnee Confederacy. Deprived of its major source of weapons, the Native alliance would be unable to resist American expansion to the west.
Most American politicians and military leaders felt that the conquest of Canada could be accomplished easily. The British had only 1,500 troops for the defence of Upper Canada: the majority of the British army was committed to the war against Napoleonic France. At the same time, the Americans had 12,000 regular troops and militia along the Canadian frontier. Ex-president Thomas Jefferson optimistically predicted that the conquest of Canada would be a "mere matter of marching." Henry Clay, the Governor of Kentucky, boasted that the Kentucky militia alone could take Canada.
When the American Congress declared war on Britain, Augustus Foster immediately dispatched messages to Canada. The British government had unofficially assigned Foster to Washington to keep them informed of any important developments. Meanwhile, the American government did not act with the same haste. Although they sent dispatches to the American garrisons along the American-Canadian border, there was no sense of urgency attached to their delivery.
As a result, both Sir George Prevost, the Governor General of Canada, and Major-General Isaac Brock, commander of the British forces in Upper Canada, were aware of the American declaration of war long before the American military commanders along the border.