You’re initial instinct is correct—Germans supported the Union because of the legacy of the 1848 revolutions (this and Fenianism also motivated early Irish-American recruits). Many saw slavery as a threat to liberty writ large, and they were also very critical of the South’s ideological conservatism and aristocratic paternalism.

German-Americans also had a huge presence in the western states (what we call the Midwest now). Many lives and worked in urban industrial centers, and believed slavery devalued labor. While they weren’t outright communists, German-Americans were familiar with Marx and Engels—and if they weren’t, their work was definitely in the zeitgeist.

Germans were also strong liberal nationalists (in the 19th-century way, not the fascist way), and they saw the secessionism as anathema to those ideas. Germans were also outspoken abolitionists, and several of their German language presses were attacked during the beginning of the war (see the story of the “Baltimore Wecker.”)

Geography matters, too. The vast majority of Germans lived in northern states, but there were Germans in southern port cities . There are examples of Germans siding with the Confederacy, but they mostly lived in the south when the war started. A lot of men enlisted because they were loyal to local communities, so it’s not surprising that Germans who’d lived in Mobile for years enlisted with the neighbors. That said, there weren’t enough Germans in the South to fill out an ethnic German brigade, whereas entire divisions in the US Army had reputations as German outfits.

Also check out the Nueces Massacre in Texas, where pro-slavery confederates shot and lynched over 30 Germans during the Civil War.

If you want to look into it more, check out Civil War histories of Cincinnati, Baltimore, Milwaukee, and St. Louis (big centers of German-American culture and, this, beer).

Biographies of Carl Schurz, Franz Sigel, and Alexander Schimmelfennig.

I’d also recommend the chapter about Germans in “Civil War Citizens: Race, Ethnicity, and Identity in Americas Bloodiest Conflict” and “German-Americans and Civil War Politics: A Reconsideration of the Ethnocultural Thesis” by Walter D. Kamphoefner.

Also check out stuff about the Turner Movement. It was a liberal gymnastics movement defeated in 1848 that became super-duper pro-Union in America.

Texas is a little more unique in this regard. A couple of things:

First, Texas actually had a few large scale German slaveholders. Dr. James Kearney at UT has written on this topic.

Second, many of the Germans in central Texas did not per se flock to the Union banner, particularly in Colorado, Washington, and surrounding counties with dense German settlement. They simply remained neutral.

When Confederate conscription troops started trying to force the young Germans in to the war, things like the Nueces Massacre ensued.

Leslie Heinsoln, an armature historian from Frelsburg, Texas whose ancestors were early settlers, reports that the oral tradition of the Germans in that area indicates when conscription troops came through that area, families sent their males of age to hide in the woods. Conscription troops went in after them, never to return. He’s published this and more about the period and Germans in that part of Texas in a manuscript I published in the Colorado County Citizen earlier this summer. Search Frelsburg civil war on that site and you should find it. It was a lengthy four part series.