The whole point to forming Canada was to frustrate American attempts at both expansion and forging diplomatic relationships. While simultaneously binding the Canadian colonies closer to the Empire.

As quite a few of the prospective territories, particularly in the West though not exclusively, were doing more Business with American Companies and shipping raw goods south. Than they were with the homeland. With grumblings questioning their relationship with the homeland. Especially with what would become the Rustbelt rapidly industrializing.

And During the Civil War. Confederate Intelligence regularly operated out of Canada. In particular Montreal Canada was a hotbed of Sympathizers.

Ironically the first place Jefferson Davis went after posting Bail. Was Montreal Canada. Not the Southern States. And his Bail was posted by an interesting cast of characters including Cornelius Vanderbilt, Horace Greeley, and Gerrit Smith. Which opens up a rabbit hole all on its own. Considering at least two of them were publicly Republicans and one was a noted Abolitionist.