I've been to Canada, and I've always gotten the impression that I could take the country over in about two days.

Delivered as a comic exaggeration, the line plays on a familiar American stereotype of Canada as vast but sparsely populated, orderly, and comparatively non-militarized—therefore (in the joke’s logic) easy to “take over.” Stewart’s humor hinges on the absurdity of casual imperial bravado: the speaker’s confidence is obviously unearned and based on a shallow “impression” from travel, not knowledge. The quip also satirizes American cultural dominance and the way powerful countries can talk about smaller neighbors as if they were simple to control. The punchline depends on the mismatch between the seriousness of conquest and the breezy, tourist-level basis for the claim.