Explanation: After the failed liberal/socialist (infighting was involved) revolutions of '48 in the German states, many of the revolutionaries decided to start a new life in the US. Naturally, their inclinations were very much anti-slavery, and for that reason German-Americans remained strongly abolitionist and pro-Union during the US Civil War, and hundreds of thousands of German-Americans volunteered for service to whip the secessionist slavers. It was said (perhaps tongue-in-cheek) that some German immigrants could speak no English, except enough to ask to serve under the highest-ranking revolutionary veteran in the Union army, General Sigel. "I wants to fight mit Sigel!"
We tend to think of historical times as very low population, because the population HAS grown significantly since then. But the USA Army in the Civil War had about 2 million soldiers in total, with about 1 million serving concurrently at its peak enlistment. 100k Germans is a significant number but sounds more reasonable in context.