Reading a biography of Walt Whitman I came upon a curious passage from his 'Hospital Visits' written in 1863, documenting his visits to Union military hospitals in Washington:
The soldiers are nearly all young men, and far more Americans than is generally supposed - I should say nine tenths are native born.
Granted, I'm British so my knowledge of the ACW is limited but I find it odd that there should be any non-Americans fighting, let alone that 10% of the forces should be foreign.
Who could these foreign troops possibly be? Canadians? Mexicans?
Edit: I'm aware of Zoave regiments fighting for both the Union and the Confederacy but I was under the impression that they weren't particularly prevalent.
The Zouave regiments in the American Civil War were not particularly foreign. They wore the uniforms because some people in power thought they looked good. The impractical fashion faded quickly.
Irish immigrants represented a large part of the Union force: this site estimates there 150k in arms. It was the experience of these immigrants that formed the backbone for and gave much of the inspiration to the American Fenian movement immediately after the war. Irish immigrants now trained in combat recognized that they could be a force to reckoned with in the fight for Irish independence. So they tried to capture Canada.
Of course, the Irish weren't the only immigrants in North American, and all sorts of people were swept up in the conflict. Many naturally wanted no part of it, and I have found that the tremendous growth of the immigrant West during the early 1860s was due in part to a wish to avoid being drawn into the conflict. I discuss this process in this and this book.
There are a lot of good answers here, especially about the Fenians participating in the Civil War as practice for an Irish revolutionary war, but I'd also like to note that Canadians played a pivotal role in the Union, especially early on in the navy; Union recruiters would come to the maritime provinces of Canada to recruit experienced sailors. After the ban on blacks was lifted, black Canadian troops made up a significant minority of the "colored" regiments. And there were a lot of adventurers, soldiers of fortune, and outright frauds that participated in the American Civil War too: My favorites are Sir Percy Windham and Robert Wheat, British soldiers of fortune who fought on opposing sides (Union and Confederate, respectively) ad became commanders, only to meet each other as Windham was captured by Wheat's unit.
There's a pretty good overview of the number, character, and types of immigrants, foreigners, and other sub-alterns who served in the Civil War called "Immigrants and the American Civil War" from A&E in 1994.
There were quite a few soldiers of prominence on both sides. Franze Sigel was an ex-Baden Soldier/ Revolutionary who left after the Spring of Nations and headed a Multicultural unit. Most Notably are the large amount of French Nobility in the war. Camille Armand Jules Marie, Prince de Polignac, was a Major General in the Confederate army and known as Prince Polecat, best known for his victory at the Battle of Mansfield. The Union Employed Prince Robert, Duke of Chartres and his brother Prince Philippe, Count of Paris (both from the Orleans-Bourbon branch) for a short while before they resigned there commissions in 1862. For the most part, foreigners were scattered throughout Regiment, but there are some notables. Obviously the various Wild Geese are already mentioned, they are the majority of examples and even formed there own all Irish Brigades. There was the 58th New York Volunteers composed mostly of Poles (http://civilwartalk.com/attachments/polish-jpg.37366/). The 55th New York (Gardes Lafayette) composed of Frenchman (http://www.dontroiani.com/images/55th%20NY%20Vols.-1.jpg) (notice the french influenced (well even more) uniforms). The 39th New York (Garibaldi Guard) wears Italian dress but was actually the most cosmopolitan "Three company's each of German and Hungarians and one company each of Frenchman, Italians, Spaniards and Swiss" (http://static.wixstatic.com/media/799b8c_9fce30ee2a25429099df6ff3d2f1339f.jpg_srz_228_325_85_22_0.50_1.20_0.00_jpg_srz). The 79th New York Volunteer Infantry (Cameron Highlanders) though donning highlander garb was composed of all sorts from the isles but the majority was scots (http://k41.pbase.com/o4/72/200972/1/64433776.MqVsVTdn.79thUniform.jpg). Most of the uniforms shown disappeared as the war went on and they all wore standard dress, but they show the different cultural heritages. Of course there or alot more then just the ones I listed. P.S. Though there were many Zouave and Chassuer Regiments in the civil war they were mostly Native born and were just emulating the military style of the time. In addendum theres a lot of grey area, in the way you could classify people, Some were true foreigners, some were immigrants, and some were sons of immigrants.
Here's a very relevant article on the involvement of Finnish soldiers and sailors
Quite many served on both sides of the war, but particularly in the Union forces. All were immigrants, of course.
According to Ilmonen, the participation rate of the Finns was significant and greater than expected given the small Finnish population that lived in America at the time. Ilmonen estimates that some 1000 Finns lived in the United States during the 1860s, and of this number perhaps as many as one hundred Finns fought in the Northern Army and Navy (Ilmonen, 1919:158).
Many foreigners served in the American Civil War, particularly for the Union. As itsallfolklore elaborated upon, the Irish were some of the most well known and respected, though others included Polish and German immigrants. For example, at Chancellorsville, the 11th Corps, known for its high concentration of immigrant units, under Oliver O. Howard was the unfortunate unit to be on the receiving end of Thomas "Stonewall" Jackson's flank attack. The Federal 11th Corps crumbled almost immediately under the pressure, and the rest of the Union army derisively nicknamed them the "Flying Dutchmen" (as Dutch was the mispronounced 'Deutsch', meaning German). However, investigation by modern historians has concluded that the German immigrants in the Corps, roughly half its strength, were actually the ones who put up the most resistance.
The Union had many German units and commanders, among the latter were many veterans of the various liberal, socialist, and nationalist revolutions of 1848 in the various German states; these commanders came to be known as "48ers". Franz Sigel was among the best known, though others included Union generals Schurz, Schimmelpfennig, Weber, von Steinwehr, and Blenker.
Extremely interesting to me is that another 48er Union general, August Willich, was actually a prominent Prussian communist, one of the original colleagues of Karl Marx. He and Marx had ferocious debates but ultimately they admired each other, and in one of his later works, Marx wrote (and I'm paraphrasing here): "In the Civil War in America, Willich showed he is more than just a visionary." Willich served in the western theater, and was noted for being a brave and daring brigade and division commander whose troops admired him. At Shiloh he actually rallied one of his regiments by having the regimental band play the (at the time, still revolutionary) tune, La Marseillaise.
Irish are probably the most prolific, but there were huge numbers of Germans as well. In Missouri in particular, the battle battle lines were set along ethnic borders, with most of the German population siding with the Union and a lot of the Irish siding with the secessionists. There was also a large amount of Italians who fought as well. A large portion of the 3rd Louisiana, from New Orleans, was made up of Italian immigrants. But having non-Americans fighting in the US Civil War was very much a part of it.
