Civil War buff from Canada here. I also taught a unit on this to a high school class last year, so I'll try to be as succinct as possible. It's a pretty complex issue. Canada, and its related territory, was a vast, largely unoccupied land with strong ties to Britain. The U.S. was a powerhouse as far as accumulating and settling territory, with the idea of Manifest Destiny leading Americans to believe they had a divine right to whatever land they set there sights on. Many assumed Canada would eventually be absorbed by the U.S., like William Seward, Secretary of State under Lincoln (who was very vocal about his desire to draw Canada into the fold). The situation in Canada at the time of the Civil War was that of a serious lack of political cohesion between the fledgling provinces, which England was becoming increasingly disinterested in and wanted to pull out its financial and military aid. Coming at a time where Canada's neighbours to the south had the largest standing army on earth (over 1 million), plus the fact that the U.S. was cranky at England over the Trent affair (unlawful capturing of Confederate commissioners aboard an English vessel by a Union ship), and that Canada had been physically invaded by the Fenians (Irish nationalists who fought for the Union who wanted to hold Canada ransom so England would release Ireland), Canadian leaders knew they had to unify in order to solidify their borders, if only politically, to deter the Americans from seeing Canada as conquerable territory. It worked.