Not to be churlish, but because it was the ONLY time the British "won" against a peer competitor in the Nineteenth Century after the Industrial Revolution, perhaps?
The Russian war required the combined might of the Ottoman, French, and British empires and the Kingdom of Sardinia to prevail against the Russians (and the threat of the Austrian Empire, of course).
If coalition warfare doesn't count why even debate the Civil War? It's not like the British are going into the war without the CSA after all. So going by the track record of Britain in coalition warfare I'd call the Union doomed then.
Though your use of quotation marks around won is highly amusing. Apparently annexing the enemies territory doesn't count
And the British lost every battle in 1880-81 against the Boers.
I imagine the fact they were outnumbered in every fight might have had something to do with it. Well that and the rather remarkable lack of critical thinking Colley displayed.
Though again the relevance of a three month war against an unreinforced colonial garrison is highly questionable.
So if it takes 250,000 British empire troops 11 months to "overrun" the entirety of Boer territory (and another 20 to actually get a surrender, but oh well, that doesn't count, apparently, because the British who died in the last 20 months of the war didn't count, I guess),
The British won the conventional war in 11 months. That is not up for debate.
The length of time it took them to win the guerrilla war is irrelevant when discussing any hypothetical war with the Union. Unless you're suggesting the Union armies will just disband let the British walk in and occupy the entire United States then fight a guerrilla war against them I don't see what point you're hoping to prove other than you seem to be unable to distinguish between a conventional and guerrilla war.
Evidently you're of the opinion that Operation Iraqi Freedom and the war in Afghanistan are 100% comparable to each other.
I wonder how many troops and how much time it will take to "overrun" the US in 1861-65.
Well since absolutely no one but you has suggested this I hope you enjoy arguing with yourself.
Or how many French troops it will take to do the same to Mexico?
36,000 seemed fairly capable of doing it. I recall Juarez was pretty strapped for options until the Civil War ended. I can only imagine how he would have gone on without the Union's help.
Or Spanish in the Dominican Republic, Chile or Peru?
Because of course Spain is comparable to France and Britain. I mean they're a European power so obviously they can be copy pasted for how each nations performance would look
Look, this may astound you, but the argument is NOT that the Mexicans could defeat the French in France in the 1860s (because AZTEC EAGLES HOOAH), or the Dominicans, Chileans, and Peruvians combined could defeat the Spanish in Spain in the same decade.
Logistics matter. Time and distance matters. There's a reason history turns out the way it does, and it usually has something to deal with physics in a rational universe.
The only astounding thing is that you always keep appealing to historical determinism in these arguments. Sure you trot out distance and logistics, but this becomes much harder to take seriously when we have rather numerous examples of large expeditionary forces being sustained and supplied at even greater distances than across the Atlantic before the very decade we are discussing.