This lie was perpetrated by omission of information. I was told that immigrants were extremely important to fill soldier ranks during the war. Nothing else was said.
Immigration law in effect in the antebellum and Civil War periods was based on the Naturalization Act of 1790 and the Naturalization Act of 1795. The first limited citizenship to “free white persons of good character.” This excluded Native Americans, indentured white servants, Asians, and blacks. The second required renunciation of allegiance to any other monarch or state.
States, not the federal government, could place restrictions on entry through their ports, but these were concerned with whether the immigrant could support himself and how to track him for taxation purposes.
Swedish immigrants came for religious freedom, if they were not Lutheran, and/or for opportunity in general. They came from an homogenous culture that celebrated community spirit and sharing. The rich were expected to share with the less fortunate, but everyone was expected to work, if able. Also, families were responsible for the well-being of their members.
Because Swedes did not differentiate between the culture of their homogenous homeland and the American culture serving a diverse population, Swedes were susceptible to socialism instead of capitalism.
No Swede had seen a black person. However, they were anti-slavery by principle and would make good Union soldiers, often in an ethnic militia in which Swedish was spoken instead of English.
Germans also came in large numbers for opportunity and/or religious reasons. They were found on both sides of the Civil War, depending on which state they lived in. Some even went to Texas. Germans in the south often owned slaves. Like their American neighbors, they considered slaves legal property and fought for the Confederates.
Germans in the north did not have an easy time, though, because the same extremists who were shouting for abolition of slavery and women’s rights had also extended temperance to eradication of alcoholic beverages and stretched hard liquor such as rum and whiskey to include beer. Now, beer was not only part of the German national heritage, it was also part of their religion because they believed that God himself had given them beer for the enjoyment of life. Thus, northern state Germans were ambivalent about serving the Union cause.
Germans formed another group of ethnic militias speaking their own language, but they were on both sides of the Civil War.
Closer to the Civil War, the massive Irish immigration fled the potato famine. They were easy marks to fill recruitment quotas. How amazing to step off the boat and be offered a job with good pay!
They did not come with racial bias, but soon absorbed the northern attitude and became extremely biased against all blacks. (Yes, I have three sources.) Perhaps some of it was because it was nice to feel that someone had lower class standing than the Irish!
A group of extremely dangerous radicals entered the country in the 1850s. Monarchies in Central Europe were fighting to regain control and even to protect their lives. The monarchs won.
The radicals, including Communists, were evicted or sensibly left on their own. Some of these people came to the United States. Most of them were from Germanic states and took part in the violent uprising of 1848. The Civil War would give them the opportunity for violence.
But the states, concerned exclusively with finances, did not screen for a history of violence. Obviously, they could not reject people based on personal beliefs. So some radicals slipped in.
The radicals identified with the United States in fighting against slavery, which they saw as a freedom issue similar to what they fought for in Europe. Some outright Communists were not only Union soldiers, but officers.
The three best known Communist officers were August Willich of Prussian nobility who became brigadier general over all (German?) Indiana troops; Joseph Wademeyer (close collaborator with Karl Marx and spokesman for Marxism in the U.S.), colonel of the 41st Missouri regiment; and Fritz Anneke, who commanded the Second Wisconsin Battery of Light Artillery.
These men heavily influenced the U.S. army by providing Prussian discipline and by organizing pro-Union militias in heavily German states such as Missouri, Wisconsin, and Indiana. Along the way, they spread the Marxist message. It was often unnoticed because they were speaking German.
When Lincoln was elected the second time, Karl Marx sent him a letter of congratulations and commended him for fighting for the oppressed slaves. It is not known if Lincoln actually read the letter.