There were regiments and even whole divisions of Germans in the Union Army. Most of them had been in this country a while, but quite a few were fresh off the boat.
One of the German divisions was the first to be hit by Jackson’s flanking maneuver at Chancellorsville and started the panic that led to Union defeat.
They weren’t paid enough to qualify as mercenaries, though.
Some were there at Chancellorsville. They had some German ancestry themselves and pitied those poor “Hessian” boys, marched right into the line of fire and falling like logs, or so they wrote.
An ancestor of mine who was a Missouri Unionist named one of his daughters Sigel. Unfortunately she died in childhood.