American minority groups took part in the Civil War in important ways, especially as soldiers and sailors fighting in the war. Hundreds of thousands of immigrant Germans and Irish fought for both the Federal and the Confederate causes, most of them for the North. Around 200,000 African Americans served in the Union Army and Navy. Ethnicity and race were used as reasons and requirements in forming military units.

Bigotry and racism were the central reasons that influenced the formation of segregated African American units such as the 1st and 2nd Kansas Colored Infantry, the 1st, 2nd and 3rd Louisiana Native Guard/Corps d'Afrique (Union), the 1st Louisiana Native Guard (Confederate), the 54th Massachusetts Infantry, the 1st North Carolina Colored Volunteers, and the 1st South Carolina Volunteer Infantry. Most of these Union regiments became part of the United States Colored Troops when the Bureau of Colored Troops was formed in May, 1863. This new structure changed their numeric designations. For example, the Louisiana Native Guard/Corps d'Afrique regiments were mustered into the 74th and 75th United States Colored Troops regiments, and the 1st North Carolina Colored Volunteers became the 35th United States Colored Troops.

Many Europeans initially joined units comprised of others who spoke the same language. Examples of companies and regiments organized by Irish, Italian, German, and other distinct cultural groups abound. The Louisiana Tigers of the Confederacy (English, French, Irish, Italian, German, Spanish, and Swiss). The Union Army's XI Corps (largely German American). The Irish Brigade of the Union Army and the 10th Tennessee Infantry of the Confederacy (Irish). The 39th New York Infantry-Garibaldi Guard (Italian Americans). A number of individuals within these cultural groups were not recent immigrants or from long-established immigrant families. These individuals remained citizens of countries they returned to after their service ended. Along with recent and established immigrants, these foreign citizens are included in each cultural group when identified.