In the summer of 1861, an American diplomat in Turin, Italy, looked out the window of the U.S. legation to see hundreds of young men forming a sprawling line. Some wore red shirts, emblematic of the Garibaldini who, during their campaign in southern Italy, were known for pointing one finger in the air and shouting l’Italia Unità! (Italy United!). Now they wanted to volunteer to take up arms for l’America Unità!

Meanwhile, immigrants already in the United States responded to the call to arms in extraordinary numbers. In 1860, about 13% of the U.S. population was born overseas—roughly what it is today. One in every four members of the Union armed forces was an immigrant, some 543,000 of the more than 2 million Union soldiers by recent estimates. Another 18% had at least one foreign-born parent. Together, immigrants and the sons of immigrants made up about 43% of the U.S. armed forces.

America’s foreign legions gave the North an incalculable advantage. It could never have been won without them. And yet the role of immigrant soldiers has been ignored in the narrative of a brothers’ war fought on American soil, by American soldiers, over issues that were uniquely American in origin.

In the 1860s, Confederate diplomats and supporters abroad were eager to inform Europeans that the North was actively recruiting their sons to serve as cannon fodder. In one pamphlet, Confederate envoy Edwin De Leon informed French readers that the Puritan North had built its army “in large part of foreign mercenaries” made up of “the refuse of the old world.”

Embarrassed Northerners claimed the Confederacy exaggerated how many foreign recruits made up the U.S. armed forces—pointing to immigrant bounty jumpers who enlisted to collect the money given to new recruits, deserted, and then re-enlisted. The underlying premise was that foreigners were not inspired by patriotic principle and, except for money, had no motive to fight and die for a nation not their own.

It was not true. Immigrants tended to be young and male, but they enlisted above their quota. Many immigrants left jobs to fight for the Union, enlisting before the draft—and the bounties—were even introduced. They volunteered, fought, and sacrificed far beyond what might be expected of strangers in a strange land.

Historians have done an excellent job of retrieving the voices of native-born, English-speaking soldiers. But the voices of the foreign legions remain silent—thanks to the paucity of records in the archives, the language barriers posed to historians, and, perhaps, a lingering bias that keeps foreigners out of “our” civil war.

Why did they fight? What were they fighting for? Recruitment posters in the New York Historical Society provide hints at the answers. One poster reads: Patrioti Italiani! Honvedek! Amis de la liberté! Deutsche Freiheits Kaempfer! (Italian patriots! Hungarians! Friends of liberty! German freedom fighters!) Then, in English, it urges “250 able-bodied men . . . Patriots of all nations” to fight for their “adopted country.”

One immigrant mother gave testimony in 1863 to an antislavery convention as to why her 17-year-old son was fighting for the Union. “I am from Germany where my brothers all fought against the Government and tried to make us free, but were unsuccessful,” she said. “We foreigners know the preciousness of that great, noble gift a great deal better than you, because you never were in slavery, but we are born in it.”

Following the failed Revolution of 1848, thousands of young Germans fled to America. They took up arms in what they saw as yet another battle in the revolutionary struggle against the forces of aristocracy and slavery. “It isn’t a war where two powers fight to win a piece of land,” one German enlistee wrote to his family. “Instead it’s about freedom or slavery, and you can well imagine, dear mother, I support the cause of freedom with all my might.”

In another letter written to his family in Europe, a German soldier gave a pithy explanation of the war: “I don’t have the space or the time to explain all about the cause, only this much: the states that are rebelling are slave states, and they want slavery to be expanded, but the northern states are against this, and so it is civil war!”

So it was civil war, but for many foreign-born soldiers and citizens, this was much more than America’s war. It was an epic contest for the future of free labor against slavery, for equal opportunity against privilege and aristocracy, for freedom of thought and expression against oppressive government, and for democratic self-government against dynastic rule. Foreigners joined the war to wage the same battles that had been lost in the Old World. Theirs was the cause not only of America, but of all nations.

Don H. Doyle is the author of The Cause of All Nations: An International History of the American Civil War. He is McCausland Professor of History at the University of South Carolina. Follow him on Facebook. He wrote this for What It Means to Be American, a national conversation hosted by the Smithsonian and Zócalo Public Square.

Sign up for Inside TIME. Be the first to see the new cover of TIME and get our most compelling stories delivered straight to your inbox. Please enter a valid email address. Sign Up Now Check the box if you do not wish to receive promotional offers via email from TIME. You can unsubscribe at any time. By signing up you are agreeing to our Terms of Use and Privacy Policy . This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply. Thank you! For your security, we've sent a confirmation email to the address you entered. Click the link to confirm your subscription and begin receiving our newsletters. If you don't get the confirmation within 10 minutes, please check your spam folder.

Contact us at letters@time.com.