I've written about 'internationalism' and the Civil War in several regards, linked here. I'll specifically repost what I've written about the Civil War and Latin America, as I think it is pretty damn interesting, and also less frequently comes up than, say, the Russian fleet visit in 1863.

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In the years leading up to the Civil War, the American policy towards her southern neighbors was not particularly friendly, and in consequence, not all that well received. The naked land grab of the Mexican-American War, the various expeditions of filibusters, and just a general lack of concern for close ties with countries who represented to the US little in commercial value either for import or export was all cause for a decided ill-will emanating northwards. When the war came, Lincoln's government was suddenly a decided need to affect an about-face, and to ensure that the Confederacy didn't succeed in overtures herself (although in reality the CSA did no such thing, only trying in vain to court Mexico).

Envoys were sent out to the various Latin American nations, all with the pointed intent to try and repair relations, and to ensure that these countries would be sure to see that there was value in Union victory for them. The most immediate goal, which was met with relative success, was simply to ensure that they would do, well... nothing. With the exception of Brazil, which made an explicit declaration of neutrality - a move that while minor, nevertheless offered the South more recognition than the United States would like - the countries all chose to refrain from recognizing the conflict as anything more than a domestic matter, a better result than with many Europeans, such as France and the UK, both of whom did chose to declare neutrality.

All in all, it wasn't that hard a sell though. Slavery had been abolished in all but Brazil by then, and pro-Union sentiment was easily stirred up in anti-Slavery groups. Not all being the most stable of nations themselves, a nearby example of successful revolution appealed little to the Latin American leaders, but more importantly, the Union representatives were mostly able to paint much of the 'sins' of the past decade specifically on the South - not entirely an inaccurate picture - and that Southern victory would mean the resumption of filibustering with a vengeance as the slavocracy sought to expand itself southwards. Relatedly, and borne out when France did just that a year later, the possible demise or weakening of the Monroe Doctrine was also a selling point, not that the various countries whole-heartedly liked to think of themselves as America's backyard, but they did nevertheless appreciate that it kept out European intervention to a better degree than they could on their own.

Again though, Brazil was less disposed, with much stronger reasons to appreciate the Confederacy - one of its few remaining fellows in slaveholding. Later in the war, Dom Pedro II's remarks that "the successes of the American Union force us to think about the future of slavery in Brazil" helps to demonstrate the sentiments, and concerns, that pressed Brazil but none of her neighbors, and to be sure, it would be less than a decade later that the Rio Branco Law would begin to dismantle slavery in Brazil.

In any case though, in late 1861, the CSS Sumter, a blockade-runner, was behind a small international incident when she found shelter and replenishment in the Brazilian port of Maranhão. Diplomatic communications saw the incident blow eventually over, with the governor dismissed officially for other reasons to the satisfaction of the American Minister J. Watson Webb (Side note, he is described wonderfully by Ferris as "whose chief characteristics were the seriousness with which he took himself and his ability to write voluminous notes in what he believed to be the best legalistic style."), but nothing was actually resolved, and Confederate ships would continue to find safe harbor in Brazilian ports, diplomatic letter writing again erupting in 1863 after the CSS Alabama sheltered in Fernando do Noronha and captured several Union merchantmen that were in Brazilian waters at the time. Once again, a governor was dismissed, and although the Alabama was kept out of Brazilian ports for the time, it was again all temporary, and the Confederate raiders - or pirates are the Union would prefer they be called - continued to prowl off the coast.

The US finally struck back at all of this on October 7, 1864, with a daring nighttime raid by Napoleon Collins and his crew from the USS Wachusett, who captured the CSS Florida while it was anchored in the harbor of Bahia, towing it away as a prize while most of her crew was ashore. In the morning, a crowd attacked the Consulate building there, but the consul himself had, wisely, left aboard the Wachusett. Once he heard of the incident, Gen. Webb hurried to the Brazilian Foreign Ministry to state that Cdr. Collins had acted without authority (In the letter perhaps, but Webb had in fact been the one to write Collins about where the Florida was), and then to raise a stink about the damage to American diplomatic property. To placate Brazil, Collins would eventually be courtmartialed - with no impact in his being lauded a hero, and by agreement of Brazil, he was soon restored to rank with little delay to his career - and a 21 gun salute was belatedly rendered to the Brazilian flag in 1866 by the USS Niepsie, but all in all, it was quite worth it to the US.

Brazil - and her less amenable approach to the United States' needs in the war - also were of concern to her neighbors, giving further reason, of course, to root for the 'Stars and Stripes'. The French invasion of Mexico in 1862 was part of a larger "Grand Design for the Americas", a vision by Napoleon III for an empire to span the South and Central American lands, allied with the Empire of Brazil - even if Brazil never came into open action for it - and with the Confederacy to weaken and buffer any interference by the United States. Less substantiated rumors also placed Dom Pedro II as planning to help several members of the deposed Italian nobility escape to South America to reestablish monarchies over the various republics.

Nor was France the only country interested in flexing muscle. Spain too was interested in clawing back a bit of her lost land. The Dominican Republic was invaded in 1861 after a rigged plebescite, and both Chile and Peru had ports blockaded and some outlying islands seized. The entire venture was an unmitigated disaster, with guerrilla operations in the Dominican Republic taxing the invaders and forcing an eventual abandonment of the venture, while the Chileans and Peruvians welcomed the US' diplomatic intervention in 1866 to help Spain salvage some slight bit of honor while running from their with their tail between their legs. In the territories they still managed to hold on to, Republican and anti-Slavery sentiments further weakened control, inspiring an eventual ten year long slave revolt in Cuba in 1868. In the end, the collective efforts were such an embarrassment to the country, that it would bring down the monarchy, inspiring General Juan Prim's first unsuccessful coup in 1866, followed by his more successful overthrow of the Bourbons in 1868.

Although it all, obviously, ended in quite the terrible debacle for Spain, as it all first rolled out, it only added further evidence for the reasons to support the Union. In the case of the Dominican Republic, while leaders had quickly appealed to Lincoln for help and abiding by the Monroe Doctrine, the United States had little choice but to stay uninvolved, not only distracted, but also unable to risk alienating Spain into the arms of the Confederacy. As President Castilla of Peru noted, it was looking to soon be the "war of the crowns against the Liberty Caps" as the European powers greedily eyed the Americas, and whatever the disagreements with the US, a quick and decisive end to her own conflict was the surest way to deter further meddling. On November 14, 1864 the Congress of Panama was reconvened, resulting in several new treaties between the Latin American nations, including agreements to stand as one against 'foreign intervention'. Although not attending or officially party, the United States was sure to indicate its support.

To an extent. Disclaimer: This was written on my phone and only briefly skimmed for mistakes. Apologies for the poor structure, redundancy, and any typos. I focus nearly entirely on Britain, because I am far less knowledgeable outside of this niche.

For one, the ideas behind American democracy were still new and untested. The Civil War was passively observed as a study of American democracy. As the war dragged on (there was an expectation that it would be over almost immediately in favor of the Union) Europeans began to further question American democracy, and the nature of the situation became an important point in British foreign policy. The latter is explained later. With Antietam in 1862, Lincoln issued the preliminary Emancipation Proclamation, his promise to make the Emancipation Proclamation on January 1, 1863. This would free slaves specifically in Confederate states, which is important, because states could reenter the union or simply not secede if they wanted to keep slaves. Without even having to issue the Emancipation Proclamation for 100 more days, this brilliant piece statement took advantage of the enormous anti-slavery sentiment. To support the South would be to support slavery, and to support slavery would be to threaten a country’s salaried labor force.

Second, just before the war, much of the world textile industry was dependent on the South’s cotton, and much of the world’s textile industry was held by the British. The British labor force was largely anti-slavery and therefore tended to side with the North, especially after the Emancipation Proclamation. That said, because cheap southern cotton had fed the textile industry to such a large extent, that the South believed that a shortage of cotton exports could earn support from Europeans, and in particular, Britain.

Take this 1858 quote from James Hammond, who was then a senator from South Carolina:

But if there were no other reason why we should never have war, would any sane nation make war on cotton? Without firing a gun, without drawing a sword, should they make war on us we could bring the whole world to our feet. The South is perfectly competent to go on, one, two, or three years without planting a seed of cotton. I believe that if she was to plant but half her cotton, for three years to come, it would be an immense advantage to her. I am not so sure but that after three years’ entire abstinence she would come out stronger than ever she was before, and better prepared to enter afresh upon her great career of enterprise. What would happen if no cotton was furnished for three years? I will not stop to depict what every one can imagine, but this is certain: England would topple headlong and carry the whole civilized world with her, save the South. No, you dare not make war on cotton. No power on earth dares to make war upon it. Cotton is king.

The idea that cotton would be king, and would be enough to win the South support from England was far less realistic after 1858, yet those favoring secession and/or war continued to tout cotton diplomacy. Cotton would soon being to be sourced in large quantities from other regions, and a global economic slowdown would lead to a textile slump. The costs of supporting the South were too great.

Citizens were highly affected by the slump, of course, but was risking a war with the North worth it? Despite tariffs, the British still sold many goods to Americans, and still had enormous land holdings in Canada at risk. Furthermore, most British citizens, while not particularly fond of the United States after the Revolution, the War of 1812, Oregon Dispute, tariffs, etc., still opposed slavery. It was much easier to just source cotton from elsewhere. The British population was largely in favor of neutrality (it’s not as though the Civil War was the only thing happening in the world--Prussia was expanding and the January Uprising was taking place, for example) but notable British politicians like William Gladstone pushed for intervention. This is apocryphal, but supposedly, Lord Palmerston (important figure in British foreign policy) was close to supporting intervention as well, and reached the decision not to only after reading Uncle Tom’s Cabin. This was during the Trent Affair, in which the Union’s navy seized Confederate diplomats to Britain, John Slidell and James Mason, from a British ship. This led to a diplomatic crisis. Some feared war, including Lincoln, who resolved the conflict by releasing Mason and Slidell.

The British still made ships for the Confederates and which was a source of political tension between the two nations, but this was largely due to private, not government interests, and was addressed.

In regards to the rest of the world, I am far less informed, so I will refrain from comment.