The United States deliberately did not become a Monarchy, such as the United Kingdom, or of many of the other European nations, but instead, wrote and passed a Constitution that made this country into a Republican form of government, and thereby the vote to elect representatives as well as the President, was placed into the hands of the democratic vote. The success of America, and its reputation of having plentitude of available land, as well as opportunity, was intoxicating for a significant amount of Europeans, that had little or no opportunity, and were oppressed not only economically but also in regards to their faith.
Those that came from Europe, predominantly migrated not into the Southern States, but rather into the Northern states, which was not random, but absolutely deliberate by those so immigrating to America. While history books have written their viewpoint on the Civil War, and the reasons behind such a Civil War; a significant amount of people and even historians believe that the war was primarily about slavery and the freeing of those so enslaved, which is to a large extent, a partial truth but not the whole truth, for rather, the Civil war had many different levels and dimensions behind it, of which one of the primary ones simply came down to the prescient fact, that those that legitimately lose at the ballot box, have no legitimate recourse to thereupon take up arms and assert their right to independence; which is essentially what the Southern rebels so did, for all States that voluntarily became part of the Union, became part of an indissoluble union, which can never be torn asunder, yet the Southern rebel States did exactly that.
Further to the point, the economies of the Northern States as contrasted to the Southern States were fundamentally different. In the North, there were cities, as well as transportation via railroad and canal, industry, manufacturing, and fair opportunity for personal l ownership of land or an enterprise, via the merits of hard work, no matter, one's individual background. In the South, the riches so generated were agriculturally based, as well as being heavily dependent upon the backs of slaves, with a small elite of very wealthy owners controlling the means of agricultural production as well as the ownership of the most impressive and productive lands, in which this elite upper class, essentially ruled their domain via their political or proxy power, as well as their control of the justice and legal systems.
In European nations, in 1848, as reported by brittanica.com, there was a "….series of republican revolts against European monarchies, beginning in Sicily, and spreading to France, Germany, Italy, and the Austrian Empire. They all ended in failure and repression…" In fact, there was many a man that took up arms in the North that was a recent immigrant to America, and thereupon saw the fight against the South, as a continuation of the revolution that had previously been fought before, or had been fought by their forebears, in which they had been defeated and suffered greatly for having been defeated. This thus meant that those European immigrants that were either familiar or sympathetic to those failed European uprisings of 1848, were extremely motivated to see that the United States did not, and would not, degenerate into a corrupt Monarchy, in which, the rich plantation elite, would continue to rule the roost, by utilizing their money and wealth so generated by their exploitation and enslavement of human labor. So too, this was a fight to liberate the underclass of poor, white, ill-educated people, that did the dirty work for the plantation owners, in return of which these poor disenfranchised southerners often received nothing more than a worthless badge of honor, of being the ones that died trying to keep the white race, as seen by the Southern elite, pure and undefiled.