Invade Canada? It was done...twice:
-- 1776: Assault on Montreal and Quebec led by Benedict Arnold. Came close to success. (some great novels written by Kenneth Roberts and others.
https://www.goodreads.com/book/show/481435.Through_a_Howling_Wildernesshttps://www.amazon.com/March-Quebec-Kenneth-Lewis-Roberts/dp/0892720832-- 1812-1814 - Montreal (failed miserably) and Detroit area...lost won lost.
Oh yes...President Polk and the 54.40 or fight issue of 1844...which would have led to much of Canada west of the Great Lakes being USA. Actually it was a ploy to insure the USA had the Straits of San Juan de Fuca and Seattle as a port...which succeeded when the 49th parallel became the border. (only three great ports on the west coast of North America...San Diego, San Francisco and Seattle...Polk got them all - the one President who operated with firm Geographic national objectives.
There was some talk with the Quebec Libre stuff of the early 1960's of English speaking Canada just bailing and applying for statehood. Didn't go very far. There were also war gaming at the time which recommended Canada just disband its military and let the USA carry the load.. Canada's efforts being miniscule during the Cold War i.e. World WarIII.
