In his famous 1854 speech in Peoria, Illinois "On the Repeal of the Missouri Compromise," Abraham Lincoln noted how the Kansas-Nebraska Act had encouraged extremism in both the North and South:
"Already a few in the North, defy all constitutional restraints, resist the execution of the fugitive slave law, and even menace the institution of slavery in the states where it exists.
"Already a few in the South, claim the constitutional right to take to and hold slaves in the free states--demand the revival of the slave trade; *and demand a treaty with Great Britain by which fugitive slaves may be reclaimed from Canada.* As yet they are but few on either side. It is a grave question for the lovers of the Union, whether the final destruction of the Missouri Compromise, and with it the spirit of all compromise will or will not embolden and embitter each of these, and fatally increase the numbers of both." (Emphasis added)
http://www.ashbrook.org/library/19/lincoln/peoria.html
Challenge: actually get the treaty which Lincoln warned against put into effect. Getting a US president to demand such a treaty is the relatively easy part. Throughout the 1850s Southerners (with support from some northern "doughfaces") escalated their demands on behalf of slavery to encompass things that would have been almost unthinkable a decade earlier--e.g., a much more stringent fugitive slave law than that of 1793, repeal of the Missouri Compromise, the duty of positive federal protection (by a federal slave code if necessary) for slavery in *all* territories, admission of Kansas under a proslavery constitution its residents clearly did not want, a right of "transit and sojourn" in free states for an indefinite period, revival of the African slave trade (though this last, unlike the others, had little support outside the Deep South and was controversial even there), etc. And in 1860 the election of a fairly extreme Southerner was by no means out of the question if the election went to the House. (Breckinridge was actually something of a moderate compared to other possible candidates of the pro-Southern wing of the Democratic party in that multicandidate race.) I could see such a president demanding the treaty Lincoln talked about. Of course getting two-thirds approval in the Senate would be hard, and yet perhaps not impossible if it were part of a treaty of peace with Great Britain after a British-American war, and rejecting the treaty meant prolonging the war. Anyway, maybe the need for a two-thirds Senate majority could be avoided by a joint resolution as in the annexation of Texas. (Even then it would be hard to get through the northern-dominated House, but remember that the House did pass the Kansas-Nebraska Act and almost passed Lecompton. The old formula of "get a near-unanimous South and use patronage to put pressure on just enough northern Democrats" might still work.)
The real challenge of course is to get a British government that would *accept* such a demand. I can think of nothing short of British defeat in a British-American war that could induce it, and even then it's hard for me to see it, given that insistence on such a condition of peace would make it harder for even a clearly defeated Britain to agree to peace, and would bring about violent dissent within the US. (Perhaps the US reduces its demands to financial compensation for slaveholders who had lost fugitives to Canada?)
In OTL, btw, even without this hypothetical treaty some US slaveholders did attempt to recover fugitives from Canada by criminal extradition proceedings based on the 1833 Upper Canada extradition law; even if slavery and therefore the crime of escape were not recognized in Canada, it was sometimes possible to accuse the fugitive of some other crime, such as theft. According to Don E. Fehrenbacher in *The Slaveholding Republic: An Acocunt of the United State's Government's Relations to Slavery* (completed and edited by Ward M. McAfee) (Oxford UP 2001), pp. 103-4: "One such effort proved successful in 1842. A slave indicted in Arkansas for larceny (including [1] theft of the horse with which he had made his escape) was returned to Arkansas by order of the Canadian governor-general. This incident provoked an outcry from abolitionists on both sides of the Atlantic. Their apprehensions increased later that same year when they learned that the Webster-Ashburton Treaty contained an article providing for mutual extradition. But in the few cases that came before them, British officials usually managed to find technical reasons for refusing to extradite runaway slaves. The most notable incident took place on the very eve of the Civil War, when a fugitive known as John Anderson reached Upper Canada after having fatally wounded a Missouri farmer who tried to capture him. Arrested and held for possible extradition as an accused murderer, Anderson was eventually released on a technicality but not until his case had caused much public furor and inspired rival judicial proceedings in England and Canada." (Incidentally, Fehrenbacher also has an interesting discussion of fugitive slaves as an issue in US-Mexico relations, which I quote at
https://www.alternatehistory.com/fo...lroad-leading-to-mexico.423402/#post-15372391)
Any thoughts?
[1] Based on my reading of
http://www.dmmserver.com/DialABook/978/080/781/9780807816981.html "including" seems to be the key word. If the horse had been *all* that the fugitive in question (Nelson Hackett) had taken, he presumably would not have been returned--at least if, like Jesse Happy a few years earlier he "had left the animal on the American side and had written to his former master to tell him where to find it."