/u/Cenodoxus showed up for this earlier answer that traces the gradual evolution of the UK and US from grudging sideeye to passive allies to Allies:

• What caused the US and Britain to have such a close "friendship" and when did it happen?

I hope you'll check it out!

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http://imgur.com/CbSZQmb

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/u/Cenodoxus's reply provides a lot of good insight, but I'm puzzled as to why it basically ignores Anglo-US diplomacy before 1890, and actually gets that period wrong in a couple places.

Webster-Ashburton Treaty (1842) and Oregon Treaty (1846)

Following the War of 1812 (clearly not a great moment for Anglo-US diplomacy), the northern border between the U.S. and Canada was not agreed upon. The disputed borders led to a number of hostile encounters, such as the Caroline Incident which led to threats of war on both sides. While the U.S. and Britain had forces drawing them together, namely economic ties, even small incidents like these border disputes (which usually occurred in Maine and New York) threatened an already fragile peace between the nations. Public sentiment in the US tended to conflate these skirmishes with national honor, which manifested in two ways. On the one hand, Americans would accuse the British of not respecting their independent nationhood. Alternatively, the association with some of these skirmishes with the Canadian independence movement would sometimes lead Americans to champion the Canadian cause (Canada rebelled against Britain in 1837) as an extension of the republican "spirit of 1776," which many hoped would lead to other independence movements around the world (indeed, they believed this was already happening around the same time, in Mexico and S. America, so why not Canada?). Britain, as we can imagine, wasn't thrilled about this.

These disputes led to diplomatic negotiations that eventually produced the Webster-Ashburton Treaty in 1842, which finalized the US-Canada border east of the Mississippi and resolved a number of disputes.

There was still, however, the question of Oregon. The US and Britain both claimed huge overlapping territories, and had seen repeated border disputes since the 18th century. Everyone who took US History in high school might remember seeing the Oregon Territory on maps as jointly claimed, as well as the 1840s slogan "Fifty-four Forty or Fight!" (referring to latitude). Britain generally controlled the territory but tolerated American citizens enough to prevent war until westward settlement began flooding the area with Americans in the 1830s and 40s. This had the makings of another powder keg, but in the wake of Webster-Ashburton, diplomats on both sides got out ahead of this one and signed the Oregon Treaty in 1846 (though there's reason to believe James Polk gave up easily on Oregon so he could focus on his real target--Mexico).

A final cause of Anglo-US tension was slavery, specifically on the high seas. Britain abolished the slave trade within its empire in 1833 and urged the US to do the same. Obviously American slaveholders disagreed, fearing Britain would pressure them to end slavery altogether. Unfortunately, this meant that illegal slavers often flew the US flag on their ships to evade authorities, leading to numerous British searches of American ships, leading to a lot of diplomatic posturing. There were also other issues, like American demands for the return of thousands of runaway slaves who had entered Canada, but the Webster-Ashburton Treaty quieted enough of these issues down to make it through the Civil War, which settled the slavery issue once and for all.

Sources

• Howard Jones and Donald Rakestraw, Prologue to Manifest Destiny: Anglo-American Relations in the 1840s

• Howard Jones, To the Webster-Ashburton Treaty: Anglo-American Relations 1783-1842

Regarding the Civil War

It's true that the Confederacy tried to strong-arm Britain into an alliance by a voluntary embargo on cotton exports, but the linked reply intimates that Britain refused to cooperate on slavery grounds, then promptly switched its cotton production to other parts of the world, namely India (but China, South America, and Egypt are also part of this conversation).

Actually, Britain could initially ignore the CSA's (again, voluntary) cotton embargo because cotton exports from the late 1850s had been so enormous that Britain had enough stockpiles of cotton to last them through the first two years of the war.

While the British public was significantly opposed to slavery and thus the CSA, the government and upper classes were much more amicable to a possible alliance, even if it never happened. The eventual refusal was a combination of British public sentiment, diplomatic countermoves from the Union (namely making it very clear to the British that any assistance to the CSA would result in all out war with the United States), and to a lesser extent new cotton supplies from elsewhere. I won't overstate this--Anglo-American diplomatic relations throughout the war showed a repeated commitment to maintaining peace between the countries, and this combined with the cotton surplus to make an alliance with the Confederacy a pipe dream in the end, but these were not quiet and foregone conclusions. Continued good relations were in many ways contingent on a Union victory.

That said, global shifts in cotton production as a result of the Civil War is easily overstated. New places like India and Egypt did grow their cotton production significantly compared to previous levels, but after the Civil War ended and cotton exports resumed, the U.S. still supplied a full 2/3's of global cotton. By 1883, nearly twenty years later, the combined exports of Egypt, Brazil, and India provided just 31% of the European continental market.

Sources

• Beckert, Sven. “Emancipation and Empire: Reconstructing the Worldwide Web of Cotton Production in the Age of the American Civil War" in American Historical Review Issue 109 (Dec 2004), pp. 1405-1438.

• Howard Jones, Blue and Gray Diplomacy

• Philip Myers. Caution and Cooperation: The American Civil War in British-American Relations

Post Civil War

Once the Confederacy was defeated and slavery abolished, the US and Britain were both eager to restore good relations. This led to the [Treaty of Washington in 1871](https://en.wikipedia.org/wiki/Treaty_of_Washington_(1871), which resolved any outstanding issues resulting from the Civil War, including naval damages (some Confederate ships were constructed in Britain) and injuries to British citizens, but also other affairs like American fishing rights in Canadian waters.

This treaty tends to mark the moment when Anglo-American relations really solidified into a permanent alliance.

Source:

• Philip Myers. Caution and Cooperation: The American Civil War in British-American Relations

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