AHC - the Liberation War

Saphroneth

Banned
The requirement here is...

A major war fought for the express purpose of liberating slaves/serfs in a Great Power or Secondary Power, by a power or coalition of powers that do not hold slaves/keep serfs.
Bonus points if the target of this war is the still-slaveholding US.

No, the American Civil War does not count since the Union did not liberate its own slaves until after the end of the war.
 
In a great power? That makes it harder. It's much easier to imagine this as some imperialist tactic whereby a poor unsuspecting primitive kingdom somewhere is attacked by an imperial power on the basis of its 'barbaric customs', so that its people can be treated like possessions by foreigners rather than by each other, which is of course much more 'civilised'.

The obvious problem is that a great power is too much work to attack for a reason that doesn't actually benefit the attacking power. I think the only easy way to get around this would require the emancipatory aspect to be merely a pretext—something which might at most be the main cause of public opinion supporting the war, but not the main cause for decision-makers to advocate it. But that might not fulfil the spirit of the OP even if it does the letter.

The easiest imaginable scenario to fulfil the spirit of the OP, I think, is a 'Northern secession' type where, preferably with no nullification crisis to alter the matter of opinion, Southern "states' rights" ideas are strangled in the cradle and the slave-holders use federal power (Fugitive Slave Act & suchlike) to enforce slavery even more aggressively than in OTL; better yet, have a PoD where the Hartford Convention secession issue gets further before dying, leading to a general culture of states' rights and mistrust of the federal government in the North. Of course this would require an earlier American Civil War than in OTL, for the South to be strong enough for this set-up to work; a British victory in the War of 1812, with the British taking northern Maine and all of Wisconsin (though not Michigan; too much American presence there, both military and settler, already) and redefining a US-Canadian border around roughly the 42nd parallel N, is useful to facilitate this. Thus make the free states' position less favourable. Then we have a large slave-holding USA and a much smaller, mostly free Federated States of America (or whatever—I'll call it FSA but I don't care about the name) concentrated in the Northeast, perhaps not even with access to the Pacific. Then, provided that one accepts the argument I have often made that slavery was not abolished because of any economic inevitability but because of ideology, one can imagine the USA lasting beyond the 19th century and well into the 20th, at which point, if there aren't world wars comparable to OTL (which really isn't as hard as certain overly deterministically inclined individuals might prefer to think), one can imagine the FSA + several European powers being repulsed enough and democratic enough to launch such a war out of ideology alone.
 

Saphroneth

Banned
The obvious problem is that a great power is too much work to attack for a reason that doesn't actually benefit the attacking power. I think the only easy way to get around this would require the emancipatory aspect to be merely a pretext—something which might at most be the main cause of public opinion supporting the war, but not the main cause for decision-makers to advocate it. But that might not fulfil the spirit of the OP even if it does the letter.
I'd say that would count, after all it would be the motivating cause of the war as far as the public was concerned.
(Another possible target is Brazil, by the way - Brazil was certainly a notable secondary power in the mid 19th century.)
 
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