What would be needed for Britain to go all in against the union

The most likely way to get Britain to go to war against the the union during the ACW is a Trent war. However this would be a war neither side would really want and therefore likely to remain limited. Most likely breaking the blockade and some fighting in Canada before both sides agree to a status quo anti. What I am asking for is how would could Britain be persuaded to not only go to war, but be willing to throw absolutely everything in their power into the war. I am meaning large Indian armies invading the west coast, the entire royal navy crushing the usn that type of war where the goal is the total destruction of the union side in the acw.
 
Not through a Trent War PoD at all, I would think. The Union is far too nervous about the South to try anything stupid and daring enough to “fully awaken the lion.”

In a world where the Civil War isn’t a threat, though, the Union trying to conquer Canada for some reason would certainly cause a total British response.
 
The best approach to me seems to be some situation in Mexico resulting in France declaring war with Britain as an ally. The issue to me seems to be that Bismarck would take the chance to grab A-L and Russia would likely support the USA.

Oh, and I doubt the Brits will use Indian troops so soon after 1857.
 
The most likely way to get Britain to go to war against the the union during the ACW is a Trent war. However this would be a war neither side would really want and therefore likely to remain limited. Most likely breaking the blockade and some fighting in Canada before both sides agree to a status quo anti. What I am asking for is how would could Britain be persuaded to not only go to war, but be willing to throw absolutely everything in their power into the war. I am meaning large Indian armies invading the west coast, the entire royal navy crushing the usn that type of war where the goal is the total destruction of the union side in the acw.

There were people in the run-up to the ACW who thought that what the US really needed was a good invasion of Canada to unite everybody around a common enemy, and even people who thought that the US should just let the South go and conquer Canada as compensation. Obviously these people didn't get what they were wanted IOTL, but if for some reason the British think that a US invasion of Canada is on the cards, they might well see the southern rebellion as an excellent opportunity to take a threatening rival down a peg or two and join in on the Confederate side.
 
The best approach to me seems to be some situation in Mexico resulting in France declaring war with Britain as an ally. The issue to me seems to be that Bismarck would take the chance to grab A-L and Russia would likely support the USA.

Oh, and I doubt the Brits will use Indian troops so soon after 1857.

The difficulty is that Britain didn't want a French Mexico any more than the US did. Britain bailed on the whole Mexico endeavor as soon as they realized what France had planned. The US and Britain were antagonistic in that period but if it came to choosing between the US and France I'm not so sure that Britain wouldn't side with America.
 
The most likely way to get Britain to go to war against the the union during the ACW is a Trent war. However this would be a war neither side would really want and therefore likely to remain limited. Most likely breaking the blockade and some fighting in Canada before both sides agree to a status quo anti. What I am asking for is how would could Britain be persuaded to not only go to war, but be willing to throw absolutely everything in their power into the war. I am meaning large Indian armies invading the west coast, the entire royal navy crushing the usn that type of war where the goal is the total destruction of the union side in the acw.

The only thing you need change is the Prince Consort Albert dying sooner, the British were fully prepared to go to war and it had popular support for such. The Prince Consort was, however, able to successfully calm the crisis by giving the Lincoln Administration an opening to resolve the situation in the British diplomatic discussions with them.
 
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