Academic scholarship lends credence to the theory that expansionist motives played a part in the frenzy for war with Great Britain among Southern and Western politicians.
While crucial maritime issues formed the bulk of American grievances against Great Britain in 1812, and these are copiously detailed by President Madison in his war message to the Congress, the ancillary issue of expansionism, particularly with the view of taking Canada, cannot be discounted. Historian Paul Johnson, in identifying this cause,  states, that the “South and the burgeoning West favored war for imperial reasons…they thought of appropriating…British Canada.” Albert Weinberg  writes that, “It has been plausibly argued by Professor Pratt  that this war, long explained by reference to impressments and commercial restrictions, was caused fundamentally by the desire of Western States for the annexation of Canada.”
From Their Own Words
Samuel Taggart’s June 24, 1812 speech opposing the war was never given during the closed-session vote, but it was published in the Annals of Congress. Taggart, a representative from Massachusetts, devotes his final paragraphs to the issue of Canada. “For whose benefit is the capture of Canada,” he asks. “What advantages are we likely to reap from the conquest?” That Canada was to be an indemnity for maritime grievances is addressed earlier when Taggart says, “Canada must be ours; and this is to be the sovereign balm, the universal panacea, which is to heal all the wounds we have received either in our honor, interest, or reputation.”
Donald Hickey, whose causes for the war focus on maritime issues, allows that, “advocates of war also hoped to put an end to British influence over American Indians by conquering Canada…”  and begins his third chapter with a speech by John Randolph [December 16, 1811] in which Randolph says, “Agrarian cupidity not maritime right, urges war. Ever since the report of the Committee of Foreign Relations came into the House, we have heard but one word…Canada! Canada! Canada!” That certain members of Congress harbored thoughts of acquiring Canada seems indisputable.
Weinberg applies the term “geographic predestination” to the expansionist elements present at the start of the War of 1812. He cites Representative John Harper (NH) as stating, “it appears that the Author of Nature has marked our limits in the south, by the Gulf of Mexico; and on the north, by the regions of eternal frost.”  Taking Canada served several purposes. Canada would be an appropriate reparation for the economic ills suffered because of British policies such as the Orders in Council, but would also fall within the natural and inevitable expansionist mode that, as Weinberg argues, had been a part of American land lust since the first days of colonization. Additionally, the British, through Canada, were thought to be behind incessant Indian raids along the frontier. Walter R. Borneman writes that, “ Thoughts of quelling Indian influence for good and ousting Great Britain from Canada became the rallying cry for Henry Clay and…the ‘war hawks.’”  This feeling was boosted by Tecumseh’s attempt to rally disparate tribes against frontier settlements in 1811.
Expansionism may have been a powerful underlying reason some political leaders supported war with Britain in 1812. Notwithstanding years of impressments, trade disruptions, losses of cargoes, and commercial strangulation, the lure of Florida and Canada cannot be discounted, if nothing else than a fitting remuneration for years of turmoil and loss. Expansionist motive was often cloaked by public and patriotic reasons that the citizenry can more readily accept and react to. More than just a theory, enough evidence exists that Canadian annexation figured into the overall strategy of war in 1812.
-  Paul Johnson, The Birth of the Modern: World Society 1815-1830 (New York: HarperCollins, 1991) p. 10.
-  Albert K. Weinberg, Manifest Destiny: A Study of Nationalist Expansion in American History (The Johns Hopkins Press, 1958) p.52ff.
-  Donald R. Hickey, The War of 1812: A Forgotten Conflict (Urbana and Chicago: University of Illinois Press, 1989) p.47.
-  Annals of Congress, 12th Congress, 1st Session, col. 657.
-  Walter R. Borneman, 1812: The War That Forged A Nation (New York: HarperCollins, 2004) p. 28.