Approximately 516,000 (23.4% of all Union soldiers) were German Americans; about 216,000 of these were born in Germany. New York supplied the largest number of these native-born Germans with 36,000. Behind the Empire State came Missouri with 30,000 and Ohio with 20,000.
Scores of individual regiments, such as the 9th Ohio, 74th Pennsylvania, 32nd Indiana (1st German), and the 9th Wisconsin Infantry, consisted entirely of German Americans. Major recruiting efforts aimed at German Americans were conducted in Cincinnati, St. Louis, and Milwaukee, among many other cities.
Commonly referred to as "Dutchmen" among other Union soldiers, and "lopeared Dutch" to Confederates, German-American units in general earned a reputation for discipline and ruthlessness. Most of them were farmers or miners. Many of the Germans who fought for the Union during the Civil War had been professional soldiers in Germany and had since come to America, many to escape persecution. The revolution of 1848 meant a lot of refugees. Europe was at peace, for the most part, in the early 1860s. Professional soldiers were out of work or stuck in the same rank for a long time. The American Civil War offered them a new venue.
Adapted from Wikipedia