I'm reading David Detzer's Allegiance: Fort Sumter, Charleston, and the Beginning of the Civil War. On page 70, he states:
During the Civil War, as an agent of the Confederacy, he [Jacob Thompson of Mississippi, and Buchanan's former secretary of the interior] would work in Canada, developing plots to swoop across the border and free Southern prisoners or to burn Northern cities like New York, activities requiring a man chary about revealing himself.
Did these plots ever come close to posing a real threat to major cities just south of the Canadian border?
What else do we know about these 'developing plots'?
Was the atmosphere between Canada and the United States strained in a way as to make these plots feasible?
With what sort of entities did Thompson find a sympathetic ear?
What did he (and by extension, the government of the South) promise?
Proper citation:
Detzer, David. "Twilight of the Old Union. Allegiance: Fort Sumter, Charleston, and the Beginning of the Civil War. 1st ed. New York, NY: Harcourt, 2001. 70. Print.
Edit: Added a line of inquiry; formatting.
Some vital context when considering the relationship between the US and Canada during the American Civil War: it ran through London. Canada was of course still a colonial possession of Great Britain at this time, so foreign relations were formally and practically handled out of London. I can't speak to the plots specifically, but there is an interesting diplomatic history between the South and the European powers, with the South continually looking to gain foreign recognition for its cause. Anything organized with tacit support from the colonial Canadian government or from the British crown would certainly have been looked at as hostility against the US at this time.
The Federal government made it known early on that any attempt to recognize the Confederate government as a sovereign power would be an act of war upon the United States and in such a war, Britain would have much to lose. William H. Seward, the US Secretary of State, made no secret of the fact that he believed the Southern crisis (as it was known in the beginning) would be a perfect opportunity to initiate the US conquest of Canada, saying that a foreign war was “the best means of establishing internal peace.” Indeed, the British War Office planned for the inevitable loss of Canada in any war with the US, while their offensive strategy relied on a blockade of the North to gradually force the Federals to capitulate.
The South's major carrot in their negotiations with the British included the fact that at the start of the war, 80 percent of Britain’s cotton supplies came from the South. And the Federals instituted a blockade of all shipping to and from the South, cutting off cotton exports. Britain was at the time the world's largest producer of textiles, so this was a big deal to them. Undoubtedly cotton supplies were less important to Canada, but it was British MPs who were making the decision about whether Canada should aid/recognize the South, not Canadian ones!
Cotton shortages caused significant labor strife in Britain during the early years of the war and Britain and France considered a joint effort to mediate the conflict on the basis of Southern separation and recognition after the Union defeat at the Second Battle of Bull Run. However, Antietam put an end to that talk. Then after the Emancipation Proclamation was issued there was no further real support for intervention on behalf of the South in either Britain or (I would presume) Canada. Abolition was immensely popular in both countries, so the constituency mitigating in favor or Southern recognition lost all favor after Lincoln framed the war in moral terms.
"King Cotton Diplomacy" is of course the seminal work in this field, and there's also: Brian Jenkins. Britain & the war for the Union: Volume 1 (McGill-Queen's UP, 1974) and D.P. Crook The North, the South and the Powers. (John Wiley & Sons, New York: 1974)
edit: corrected to "Second Bull Run"
Excuse me, but wasn't New York one of most pro-Confederacy city in North with draft riots? Why would confederacy attack New York?
It wasn't New York, but a couple dozen Confederate agents did come into St. Albans, a small town in northern Vermont, from Canada in 1864. They robbed three banks, claimed to have conquered the town, stole some horses, and rode back towards Canada. However, the townspeople telegrammed the Canadian border after the Confederates left, and most of them were captured when they got back to Canada. and yet, the Canadians had trouble deciding whether this was a crime deserving punishment, or an act of war in which a neutral country like Canada/the UK should not interfere. After arresting, freeing, re-arresting, re-freeing, and re-re-arresting them, the Canadians eventually freed the people but gave the money back to the banks (though some of the money, along with some of the men, were never captured and presumably got back to the Confederacy).
This was a somewhat wiley move because the war was going poorly for the Confederacy at this point, and they were in need of money and morale boosts, not to mention demoralizing/confusing the Union. They were getting desperate and willing to try more creative approaches to furthering their cause. The biggest effect was to sow fear among the northerners.
For more, see this episode of the Futility Closet podcast, and the books, newspaper articles, and other things the podcast cites.