The United States declared war on Britain on 18 June 1812. Despite his preparations, Brock was worried about Canadian security. In Upper Canada, besides the militia, there was only one British regular infantry regiment, a detachment of retired veterans, and a company of artillery. These had to be dispersed among several widely separated posts. Brock's advantage was that the armed vessels of the Provincial Marine controlled the lakes, and allowed him to move his reserves rapidly between threatened points.
Although Brock's achievements in Canada were overshadowed by larger-scale fighting in Europe, his death was still widely noted, particularly in Guernsey. In London, he is remembered at a memorial in St Paul's Cathedral. This was paid for by £1575 voted by the House of Commons, which also granted pensions of £200 to each of his four surviving brothers. For his actions in the capture of Detroit, Brock was appointed a Knight Companion of the Order of the Bath (KB) on 10 October 1812. He died at the Battle of Queenston Heights before learning of his knighthood.