theroyalcouple said: Huh? The British signed a decree of neutrality in 1861 when it came to the civil war. No difference of opinion either way.
The civil war had no bearing on Britain being an ally or not
Maybe you haven't read the same history books concerning diplomatic relations between the UK and US during the Civil War that I have. At the outset of the war, British cotton mills in the Midlands needed Southern cotton (other sources had not yet been developed), and the UK was not certain that the United States would survive intact if the Civil War continued, so while they may have been officially neutral, on the more practical level they were playing both sides of the coin for a while, and from a Union perspective, they were aiding our Confederate adversaries and enemies.
The British government also allowed the UK-backed funding of the Confederate commerce raiders, including the C.S.S. Alabama, C.S.S. Shenandoah, and the C.S.S. Florida, UK built ships that destroyed millions of dollars of Union maritime commerce, and the issue was only settled during the Alabama Claims case in 1871. Also, the UK government was aiding the Confederate economy by continuing to purchase cotton that had slipped the blockade, and this also caused friction between the Union and the UK, until Gettysburg, when the UK government realised that the United States would probably not split, the Confederacy had no chance of winning, the Union would undoubtedly win the war, and the UK would have to deal with a growing and more powerful United States government as soon as the war ended. After Gettysburg they became more overtly friendly to the Union and started giving the Confederacy the slip. By this time, alternative sources of cotton were developing in Egypt and other regions.
Also, increased Union production of industrial products, including armaments, factored into the relationship. The UK government did not want to be facing a future potentially hostile, heavily armed, economically and industrially powerful 'United' States. Their leaders weren't stupid.
As we know, declarations of neutrality often are for face value only. The US famously was "neutral" at the start of WW1, but the reality was otherwise.
Such are the vagaries of geopolitics.
So that is why I mentioned that during the Civil War there was a time where the US and UK, although still cordial to each other, weren't exactly on friendly terms. The earlier years of the US Civil War were probably the only time since the War of 1812 that the US and UK were not on great terms.
That said, UK politicians during the 1800's were amazingly prescient, always looking out 50-100 years in the future. That's why they did not use the Royal Navy (or diplomatic means) to try to break what they believed was an illegal Union blockade of the Confederacy. Instead, they let it pass. This was even though the cotton mills in the Midlands started hurting because of the effectiveness of the blockade.
Then, 50 years later, when the US complained about the UK blockade of Germany in WW1, the UK government said "We are just following the precedent you set."
PS I don't mean to sound like Mr. know it all here, but I'm just trying to explain what I meant upthread. There were indeed some diplomatic difficulties between the US and UK during the Civil War.