Not many people are aware of the role the British played in the American Civil War, or that even they HAD a role in that conflict. In grad school I studied an event called the Trent Affair and wrote an exhibit proposal to portray this immensely important moment in American and world history. I'm posting an excerpt here of the background information on the Trent Affair, to give people a good idea of how close Britain came to fighting alongside the Confederacy (despite a few conflicts of interest) -- after you read it, think about how differently the American Civil War would've ended had Great Britain invaded the Union from the North and decimated its Navy in the Atlantic... (also, please forgive the formatting and footnotes).
The Trent Affair and Anglo-American Relations during the American Civil War:
Diplomacy Determining the Future of a Nation
On November 8th, 1861, two Confederate diplomats, James Mason and John Slidell, were on the British mail ship the RMS Trent, on their way to Great Britain and France to plead for assistance from Europe on behalf of the South. Union intelligence was well aware of this plan and had sent a United States sloop-of-war, the San Jacinto, to intercept the Trent, firing two shots over its bow in an attempt to make it stop. The captain of the San Jacinto, Charles Wilkes, had categorized Slidell and Mason as “contraband” and therefore necessary to be removed from the Trent, but they initially refused to be escorted off the vessel by Lt. D.M. Fairfax. Commander Williams of the Trent “thereupon made the following protest:”
In this ship I am the representative of her Majesty’s Government, and I call upon the officers of the ship and passengers generally to mark my words when, in the name of the British Government, and in distinct language, I denounce this as an illegal act, an act, in violation of international laws – an act, indeed, of wanton piracy, which, had we the means of defence, you would not dare to attempt.
The Confederate diplomats were eventually led by Fairfax’s men to the San Jacinto’s cutters and jailed in Fort Warren, a prison for captured Confederates in Boston. News of the interception reached Great Britain; there was a great public outcry, condemning the audacity of the American vessel had in firing upon and 
boarding the British vessel in a time of peace. In response to the bold move from the North, Britain sent 11,000 troops to Canada and prepared their navy for battle, should the Union not issue an official apology for the incident and promptly release Mason and Slidell. The British forces were also sent to Canada for another reason; ministry members of Great Britain “began to worry if the American response” to their demands “might be a sudden invasion of Canada.” Professor Jonathon Winkler of the Wright State University history department, specializing in this particular field of history, states in a personal correspondence that the British were particularly concerned with the relatively undefended border between Canada and the United States. The prospect of fiscally supporting the military forces in a war against the Union was troubling to many members of the British Parliament, as well as the idea of having to sustain those forces for a battle that could go on for years. There were also many political implications in a war with the Union; Great Britain had been staunchly opposed to slavery, and to support the Confederacy, which had practiced slavery, seemed to be a conflict of interests.
Despite the impressive odds stacked against them, Union soldiers were eager to fight the British again. They were resentful of their continued influence in America, as well as embittered about the War of 1812. Charles Francis Adams, the United States minister to Great Britain during the time of the American Civil War, wrote of his personal account regarding the international incident in The Trent Affair: an Historical Retrospect. He states that the feeling of Americans “had slowly been fermenting to one of acute hostility towards Great Britain,” for several reasons. Firstly, Britain had seemed “somewhat Pharisaic” in its attitude towards the United States and had “done nothing to forward the cause of the Union in a crisis brought on by the aggressive action of the South,” despite the shared anti-slavery sentiment shared between the Northerners and the British. In addition, British newspapers, such as the London Times, “had been distinctly unsympathetic, not to say antagonistic and otherwise acutely irritating,” Adams notes in his reflective work. His comments were fairly accurate; the London Times was particularly hostile towards the Union. An article in the newspaper, after the Trent affair had occurred, states it became “of momentous interested to [the British] to know how high this flood of ignorance and passion has risen” in the United States, regarding the eager reaction of some Americans to the prospect of a war with Great Britain.
Norman Ferris describes the response of the American historian Henry Adams when faced with the idea of a war with the British. He states in his book, The Trent affair: a diplomatic crisis, that the man “declared with boyish bravado that since England was waiting only for the right moment before attacking the war-weakened United States. . .” he believed “it had been wise for the Americans to strike first with an act which was virtually a declaration of war.” When British diplomats heard of this seemingly-popular sentiment, “the news was hurriedly telegraphed to the minister and the secretaries settled down nervously to await his return.”
Northerners were not the only ones bracing themselves for war; tensions were growing on the other side of the Atlantic as well. Ferris writes about Prime Minister Palmerston sending a letter to the British War Office, suggesting that the reductions in British military spending for 1862 be reconsidered. “‘Relations with Seward & Lincoln are so precarious,’ wrote the premier, ‘that it seems to me that it would be inadvisable to
make any reduction in the amount of our military force.’” A December 21, 1861, article from The Illustrated London News states that the British public learned “with something akin to disgust that the barbarous reprisal system is likely to come into effect,” and that British citizens could expect that Mason and Slidell were being treated cruelly, and “may actually be executed in cold blood.” The author of the article then comments on the Union’s destruction of Southern harbors and other savage acts as being an “excess of zeal for civilization;” this is indicative of British public feeling at the time.
Taking the advice from men, including former president Millard Fillmore, President Lincoln was well aware that a two-front battle would mean dire results for the Union, especially against an enemy as strong as Great Britain. He ordered the release of the two captured diplomats, as well as formally disavowing the actions of Captain Wilkes and offering reparations to the British. Horatio Nelson Taft, an examiner for the United States Patent Office, writes in his diary on December 28, 1861, about the release of Mason and Slidell:
A good deal of excitement in the City in reference to the surrender of Mason and Slidel [sic]. I suppose it was the only way to avert a war with England, which at this particular juncture would be an increase of business for the Country not very desirable. But a War with England on the "Trent affair" question would be very popular and unanimously supported by the Country. We shall have to have a fight with England before long, no matter how soon, after the Rebellion is crushed out.
Diaries from regular citizens like Taft, who had no political or diplomatic connections, offer insight into popular American public thought about the Trent affair. As Taft declares himself, a war with Great Britain would, at the time, be supported by much of the Union (though perhaps not as unanimously as Taft had suggested).
In the end, Mason and Slidell continued their voyage to Europe but were unable to achieve diplomatic recognition, and the crisis of the Trent affair ended quietly. As the American Civil War progressed, Great Britain remained neutral, and its impression of the North softened significantly by the end of the conflict. Arthur James Lyon Fremantle, a British Lieutenant Colonel, travelled to the Confederate States in 1863 to witness the war first-hand. In his preface, he comments
that, “at the outbreak of the American war, in common with many of my countrymen, I felt very indifferent as to which side might win; but if I had any bias, my sympathies were rather in favor of the North, on account of the dislike which an Englishman naturally feels at the idea of slavery.” Fremantle’s attitude towards the war is indicative of the shift in the British public opinion on the American Civil War; two years prior, much of Great Britain, especially the aristocracy, had hoped to see the South triumph over the North. As memories of the Trent affair faded, so, too, did the bias against the Union.
In 1864, President Lincoln received a letter from a British man named John Campbell, who declared his support for the reelection of the American leader. Campbell writes that “multitudes of candid, reflecting, patriotic and humane men in Great Britain” supported the re-nomination of Lincoln for a second term as president. With his letter he sends a copy of the British Standard to further express his and his country’s appreciation of Lincoln’s “mighty work, [which] under an accumulation of obstacles, such as for variety, complexity, and magnitude, has never before surrounded the Ruler of any nation.” This shows that, in the final years of the American Civil War, British opinion became favorable towards the Union; the man with whom Prime Minister Palmerston had “precarious” relations only three years prior was now seen as an exemplary leader.
boarding the British vessel in a time of peace. In response to the bold move from the North, 
make any reduction in the amount of our military force.’”[7] A December 21, 1861, article from The Illustrated London News states that the British public learned “with something akin to disgust that the barbarous reprisal system is likely to come into effect,” and that British citizens could expect that Mason and 
that, “at the outbreak of the American war, in common with many of my countrymen, I felt very indifferent as to which side might win; but if I had any bias, my sympathies were rather in favor of the North, on account of the dislike which an Englishman naturally feels at the idea of slavery.”[12] Fremantle’s attitude towards the war is indicative of the shift in the British public opinion on the American Civil War; two years prior, much of 
