It may seem weird that another country would just show up to war to have a look, but it used to be a fairly common activity, one the United Nations still practices. A military observer is a diplomatic representative of sorts, used by one government to track the battles, strategies, and tactics used in a war it isn't fighting, but may have an interest in watching — and learning from.
Professional soldiers were embedded within fighting units, but were not considered diplomats, journalists, or spies. They wore the uniform of their home country and understood the importance of terrain, technology, and military history as it played out on the latest battlefield. The Civil War had no shortage of interest from the rest of the world.
England, France, and Germany all sent observers to both sides of the fighting as early as 1862. They were concerned with the technologies related to metallurgy, rifling of cannons, explosive shells, cartridge calibers, and, of course, the new observation balloons used in the war. German observers were concerned with the power of militia and volunteer forces in the face of a standing, professional army. These observations formed many of the tactical developments used in later conflicts, especially World War I.
The Prussians, with an aforementioned interest in the superiority of professional armies, didn't think much of the armies fighting the war. While noting the tactics used by American fighting men, Prussian observers thought the New World's way of war was inferior to the Prussians'.
One Prussian captain, Justus Scheibert, divided the war into three phases. The first was made up of the disorganized skirmishes. At this point, neither side had really come to grips with the war and their own strategic capabilities. The second phase, which ran from 1862 through the 1863 Battle of Gettysburg, was defined by a refinement in battlefield formations, which were used to great effect by both sides. After Gettysburg through to the war's end, the fighting became defensive for both sides, where belligerents fought for inches of battlefield instead of mounting a great retreat or advance.
Scheibert believed that the construction of defensive fortifications that allowed officers time to make careful decisions replaced the skill of trained professional officers in quick decision making. Like many historians in the decades following the war, he cited Union manpower and industrial output as the chief tools of victory for the war while praising Confederate General Robert E. Lee for his innovations that allowed Confederate troops to stay relatively fresh and punch above their weight class, even when outnumbered.
The British, meanwhile, were horrified at the war's destruction and bloody death toll. The British government wanted the horror to stop and felt compelled to pressure the United States to accept a negotiated, two-state solution. London could not understand Lincoln's motivation for keeping the Union together by force in a democracy where people are supposed to be able to determine their own futures by voting. Neither Britain nor France understood why the North and South both rejected publicly making the war about its central cause: slavery. They simply did not understand the politics of the U.S. as well as President Lincoln and did not understand the Confederate government's chief fears as Jefferson Davis saw them.
London was also turned off by the Confederate threat of an embargo of cotton exports to Great Britain. It turns out they played this hand much too early, as British merchants would seek alternatives and replacements for Confederate cotton as early as 1861. But as the level of death and destruction rose, both Britain and France began to plan to intervene for the South. Even Lincoln's Emancipation Proclamation angered European powers, who saw the limited emancipation as nothing more than an attempt to incite a mass slave uprising to save face in losing the war.
The only thing that saved the Union from a combined French-British intervention was the risk or war with the United States and that the South had not yet proven that it could fight the Union Army to a greater defeat on the battlefield.
One British observer actually visited nine of the eleven Confederate States during the war. Arthur James Lyon Fremantle, just 25 years old, took leave of the British Army to travel to Texas via Mexico, moving through nearly the whole of the Confederacy, He met Generals Lee, Bragg, and Longstreet, to name the most important, along with Confederate officials, including President Jefferson Davis. After observing the Battle of Gettysburg (where he met the Prussian Captain Scheibert), he crossed the lines and moved north to New York, where he left for home.
The Britisher remarked that Texas was the most lawless state in the Confederacy, that even Confederate generals were notably impoverished, but were in such good humor that they could ride their confidence into battle. As for the generals themselves, he thought it was amazing that a general like Longstreet would lead men into full-frontal assaults, and that a man like General Lee would speak to individual troops while taking responsibility for the losses on the field.
The French were interested in a Union loss and the creation of a new republic, carved from the remnants of the United States because they were determined to recoup the losses suffered at the hands of the British during the colonization of the new world. France's criteria for intervention were much the same as Britains, but were dashed after the Union victory in the war and any preparations made to use Mexico to capture former French territory west of the Mississippi were scrapped.
Though the world's other powers didn't think much of the war and its fighting for the duration, the preparations they all made throughout the war and in the years immediately following shows the lasting impact it had on global politics. In all, visitors from Germany, Britain, Italy, France, Russia, Nicaragua, and Austria all visited various battles of the war. The lasting legacy of this impact is the continued debate over what might have been, even more than 150 years later.