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I get the feeling you're reading my posts to a certain point and no further.
I agree that there was no benefit to Britain for fighting a war with the Union in the Civil War as it developed.
I however do feel that there would have been benefit for Britain fighting a war (at least that the British decision makers would have seen a benefit) in the event that the Union had not backed down over the Trent Affair.
It also seems to me to be distinctly odd that you seem to hold there are only two positions for the Union's resilience to defeat - either they won't negotiate unless they're completely occupied, or they're "cowardly low lifes".
I'm actually arguing for neither, I'm arguing that the Union would continue fighting for several months while they still see a way out of the war but that they would eventually be compelled to negotiate when they saw a major collapse in warfighting capability approaching, caused by a combination of a serious lack of strategic materiel and a financial crisis.
(This is consistent with the way wars tended to end between Great Powers and even with secondary powers in this period. It's also what led to the end of the American Revolutionary War, so it's a good model for a successful independence war in which an outside conflict also takes place.)
As to the point about slavery, it's factually correct for 1862 and it's important when understanding British points of view - they were so anti-slavery they disliked the North for it as well.
Perhaps I should provide a few citations.
Gladstone:
"we may have our own opinions, and I imagine we have our own opinions about the institutions of the South- ('hear hear,' and applause)- as unfortunately we may have our own private opinions about the countenance that has been given to those institutions in the North- ('hear hear' and applause)... Why, no doubt if we could say this was a contest of slavery or freedom, there is not a man in the length and breadth of this room- there perhaps is hardly a man in all England- who would for a moment hesitate upon the side which he would take- (hear, hear)- but we have no faith in the propagation of our institutions at the point of the sword ('Hear hear' and cheers)... You cannot invade a nation in order to convert its institutions from bad ones into good ones, and our friends in the North have, as we think, made a great mistake in supposing that they can bend all the horrors of this war to philanthropic ends. (Hear, hear). Now, gentlemen, there are those among us who think- and I confess, for one, I have shared the apprehension- that if in the course of the vicissitudes of the war the Southern States of America should send an embassy to Washington, and should say, 'Very well; we are ready to lay down arms... upon one condition- that you shall ensure us that there shall be no interference with our domestic institutions.' Ah, gentleman, we have had a fear that that application, if it were made, would receive a very favourable reply. ("Hear hear", and cheers)." (Liverpool Mercury, 25 April 1862)
Newspaper reports on the British reaction to the contraband policy:
"Slaves [Frank Baker, Shepard Mallory and James Townsend] have fled to the camp of General Butler; and when their owner, under a flag of truce, claimed their restoration, Yankee ingenuity raised the curious pretence that they were 'contraband of war', but said they should be restored on the owner taking an oath of fidelity to the Union... After great perplexity, the Cabinet of Washington has instructed the Commanders to receive escaped slaves and set them to work, keeping an account of their work and the cost of their keep. Is it thus that men make war to put down slavery?... what is 'the anti-slavery sentiment' that, instead of saying to these fugitive slaves- 'Go forth; we shall do nothing to return you to bondage;' detains them, keeping an account of their food and earnings, that a balance may be duly made when they shall be returned to their former owners, or sold to reimburse the Federal treasury? Dr Beecher and Mr Phillips had better teach 'the anti-slavery sentiment' at Washington, before they trouble themselves to cross the Atlantic. Our 'anti-slavery sentiment' tells us to scorn this miserable paltering. Providence has destroyed, by the appalling judgement of civil war, the old devices by which the Free States propped up the system of the Slave States; and even in the midst of that war, the men who say they are fighting for liberty, actually embarrass themselves with the care of the human chattels in the interest of slave owners. We console ourselves with the belief that this shallow expedient will break down. The army may take charge of a few hundred slaves, but it can do nothing with them when they come forth by thousands; and the movement of slaves having commenced, it must go on spreading and strengthening while the war continues." (Sheffield Independent, 15 June 1861, p. 7)
“They have proclaimed theirs to be the land of freedom, while they have become utterly oblivious to the fact that their Union involved a system of slavery more cruel, degrading, and ****ing to the human feelings, intellect, and spirit, than ever before disgraced the world… Do the Northern States seek to free themselves from these heavy charges? Hypocrisy impotent as contemptible! Where under the canopy of heaven did colour stamp a man with such hopeless misery as in the streets of New York, Philadelphia, or Boston?... The triumph of the South cannot make Slavery worse; the triumph of the North can hardly make the position of the slave better, when even now she designates him as a 'chattel', and talks of him as being 'contraband of war'.” (Huddersfield Chronicle, 13 July 1861 p. 5)
"Another piece of news brought by the last steamer, is the remarkable proclamation which General Fremont has issued in Missouri... The slaves held by rebels are, by this proclamation, declared to be free, and not 'contraband of war', as has hitherto been the case. This is a most important distinction, and we regard it as the first step towards making the present struggle a war of emancipation... A movement of this kind will not be easily put back... We are thankful that the patriotic Fremont... has had the courage to act as he has done, and we trust that before long, the principle which he has thus broadly and publicly avowed, will obtain the enlightened and energetic support of the Federal Government." (York Herald, 21 September 1861, p. 8)
“It is certainly stretching the doctrine of contraband of war very far… the argument is as absurd and untenable as an argument could possibly be… when the necessity of emancipating the slaves is so strongly felt that people are ready to seize upon the most obviously absurd pretext as reasons to justify it, it is evident that the day of action is drawing nigh. We have always anticipated its advent, and are not at all surprised to see it coming so soon, nor sorry to see it coming with such ridiculously awkward excuses.” (Leeds Mercury, 8 October 1861)
"Mr Lincoln- long the chosen representative of Illinois, a State which has always signalised itself by a reluctance to allow of the settlement of free negroes on its soul... It is probable that even at the seat of the Federal Government no one is able to tell exactly what becomes of the 'contrabands' who flock to the camp of the army of the Potomac, and few persons, perhaps, feel much curiosity on the subject. Every military officer is allowed carte blanche, and follows his own lights in the matter. Wherein it is observable that those belonging to the regular army generally show a disposition to pay more attention to the vested rights of the master than to the inherent rights of the fugitive." (Bradford Observer, 6 February 1862, p. 7)
And more generally:
“Not only slaves but free coloured men were treated with the greatest ignominy in the Northern States... From the Northern churches the people of colour were practically excluded; they were treated with a contumely which was more insulting even than the direct tyranny of the South... He believed that the North would willingly give up not only the four millions of slaves, but the half million of free-coloured people resident in the North, if by that means the reconstruction of the Union could be effected.” (William Howard Day, Sheffield Independent, 17 January 1862)
“Wherever you find a black person in the Northern cities of America he is most wretchedly treated; he is treated worse than you would treat a dog… At this moment there are fights constantly taking place in Washington and other cities because the poor black man wishes to ride in an omnibus (Hear, hear). I said the people who would do that are not the friends of the black man (Cheers.)” (John Arthur Roebuck MP, Sheffield Independent 11 July 1865)
“The most miserable exhibition of imbecile weakness has been made by the President in addressing a deputation of the coloured people who were invited to meet him at the White House and hear his oracular utterances... He tells the coloured people that their presence in the republic is a great embarrassment... The proud, tyrannical, dominant race, who make fine professions of universal freedom and world-wide philanthropy, are the humble suitors to the despised and down-trodden coloured people, and entreat them to go.... America is as much the native country of the men of African, as those of English, Irish, or German descent.” (Sheffield Independent, 4 September 1862)
“Walter S. Cox, the commissioner under the Fugitive Slave Law, to-day [11 June 1863] remanded seven runaway slaves, two of them children, from Maryland, to their claimants. An affidavit of the loyalty of the claimants had been made.” (Leeds Mercury, 25 June 1863)
This is not to say that the British would find common cause with the South because of this; they wouldn't. The Trent affair caused great annoyance and made the British (public and government) ready for war, but they viewed it as a war forced upon them and some papers or MPs were quite angry that it might lead to some benefit for the CSA!
“England has too cordial a detestation of slavery to unite even in appearance with the South." (Leeds Mercury, 30 November 1861)
“There is not- and it is a proud boast- a single paper in the United Kingdom that, following the example of the debased New York papers, proposed to make the most of the difficulty of America, for former affronts to this country, by recognising the Southern Confederacy in retaliation for the act of Captain Wilkes.” (Bradford Observer, 5 December 1861)
"There can be no peculiar sympathy between England and the new Confederacy so long as slavery is the base of Southern institutions" (Saturday Review, December 1861)
"The gulf placed between us, between the English nation and any community that subsists by slavery, that makes it impossible that we can ally ourselves either by sympathy, community of feeling, or affection with any nation that exists upon that which I can call nothing but a disgrace and curse to mankind (Cheers). (Edward Horsman, MP, December 1861)
"We pray earnestly that God may avert from us the great calamity of finding ourselves striking hands with Southern slavery" (Eclectic Review, January 1862)
"England allied with a Confederation of slaveholding states... such a position as this would have been a cruel necessity... Thank God, we have escaped the danger." (Illustrated London News, January 1862)
In this light, the reason I focus upon Trent is that it's a flashpoint for an entirely separate war. This is much easier than any kind of intervention, and the most likely kind of intervention would be a multinational intervention to prevent bloodshed and encourage arbitration in a war that otherwise looked like being far too bloody. (This intervention would require the buy-in of Russia, the Great Power most friendly to America, so without their buy-in it's not happening.)
To recapitulate:
I think that...
...the Trent War is by far the easiest way for the British to get involved in the ACW because it is explicitly not an intervention.
...the balance of force and momentum against the US in that case would be strong enough that it would be in trouble by the middle of 1862.
...the US would sue for a negotiated peace when it was losing with the momentum against it and no clear path to victory, thus doing the same thing as any other Great Power in trouble in this period.
...this is not the US being unusually weak but is the US being unexceptional in the sense of its resilience to things going badly.
...without the Trent affair as an inciting incident to a separate war then a British intervention is very hard to arrange and requires the war fundamentally changing.
And
...that the US being vulnerable to serious trouble in fighting a simultaneous war with another industrially and navally strong Great Power on top of the historical ACW is not an indictment to the US; it's a recognition that the ACW was a hard war to fight for the Union, and that most Great Powers have a significant peacetime military. The British would have serious trouble dealing with the addition of the United States to their list of enemies in early 1915 (the comparison's not perfect but good enough to be going on with).