Okay, here's an odd thought, inspired by reading.
One of the side effects of the end of Civil War was the end of the Elgin-Marcy Treaty, which was a free trade agreement on raw materials ibetween the US and British North America.
The Treaty was ended after the Civil war; some thought it had outlived its usefulness, while Canada's own tariffs on manufactured goods (British and American) angered quite a few people.
It was poular in the midwestern farming states, however. And in a world where the mouth of the Mississippi is controlled by a foreign power that the US was just at war with, shipping goods through Canada would be a welcome prospect.
Of course there are other implications. So, let's assume that it isn't one of the typical "the war ends with the Enfield Rifles burning Washington" TLs.