Sheaffe returned to Canada from a visit to England in July 1812. The next month, the War of 1812 broke out. Sir George Prevost, the Governor General of Canada and commander in chief of the forces there, appointed Sheaffe to command the troops at Fort George on the Niagara River. While Brock was absent, dealing with an American army at the Siege of Detroit, Sheaffe was required by Prevost to negotiate an armistice with the American forces on the opposite side of the river. Prevost may have believed that peace could be negotiated quickly, but by the time the armistice ended, the Americans had been substantially reinforced.